Frame challenge.
You've got a millionaire BBEG, and you want to make him able to take on powerful foes. Your plan for this is to balance his stats so that he can become essentially a fake leveled character, and can go toe to toe against them in a straight fight for a while.
In this essay, I shall suggest you instead make him a real mastermind.
It's useful to look at the world of superheroes and supervillains, where "Rich Guy is Powerful" is a very popular trope: seems like just about every super-being is at least a millionaire, and plenty are billionaires or worse: Medusa ($1Bn), Mr Terrific ($1Bn), Silver Sable ($1Bn), The Wasp ($1+Bn), Nighthawk ($4Bn), Norman Osborn ($4Bn), Crystal ($4Bn), Angel ($5Bn), Green Arrow ($5+Bn), Blue Beetle ($10Bn), Emma Frost ($11Bn), Iron Fist ($20Bn), Mr Fantastic ($20Bn), Dr Doom ($35Bn), Ozymandias ($50Bn), Lex Luthor ($75Bn), Batman ($80Bn), Nightwing ($90Bn), Black Adam ($90Bn), Professor X ($100Bn), Sunspot ($100Bn), Iron Man (!$100Bn), Namor ($260Bn), Black Bolt ($300Bn), Black Panther ($90Tn), Aquaman ($150Tn), Kallark ($an entire galaxy) - (mostly via https://www.comicbasics.com/richest-superheroes-of-all-time-ranked-by-wealth/)
For those with no innate super powers, the way they use money varies on a scale from Iron Man to Lex Luthor.
On the Iron Man end, they use the money to essentially turn them into a fake super-being. Buy lots of gizmos to make themselves buff. A little further along the scale, there's Batman and Green Arrow. Same basic principle. That's a recipe for a super-powerful glass cannon or golem with a weak squishy dude inside. He's dangerous because you've carefully balanced his stats to match your players', so that they can have a toe-to-toe spreadsheet battle by rolling dice at each other until one falls over.
That kinda combat is definitely a valid and fun part of RPGs, and totally the entire core of tabletop war-gaming: you pay points to get balanced forces, then smash them together to find the winner.
But it's not, I'd argue, best placed as the ultimate goal of a longer RPG campaign.
So if we look way out on the other end of the scale, we find people like the Joker, or Lex Luthor. No fancy armor, just a fine suit, strong mind, and big sackfuls of cash.
Why do so many villains lie on this end of the scale? Because super-powered combat-villains feel cheap. They feel like senior henchmen, rather than like Masterminds.
How do Lex and other Masterminds avoid being killed in a finger-snap by the Heroes? By balancing their strengths against the super-beings' weaknesses, rather than trying to match and attack the super-beings' super-strengths.
Toe-to-toe turn-based combat is the primary skill of most parties. It's their super-strength. Making a BBEG commoner fight PCs head on is like having Lex Luthor challenge Superman to an arm wrestling contest.
Weaknesses of characters (and players!)
Masterminds bring the heroes low by NEVER going toe-to-toe. Not by punching them in the face, but by being prepared and informed. A well prepared rich normal person can win against super-beings if they have prep time to target the party's specific vulnerabilities, and play to their own strengths.
What's that, I hear some of you say? Your well-balanced party has NO significant weaknesses? I'd argue, then, that we may have different definitions of "weakness". Does the party have a penchant for cute things? Shiny things? Magic items? People who give them cool quests? A desire to do good? To protect people? What are their main strengths, and how can each one of them be considered a weakness?
Just like their strengths, most of the weaknesses of your party will be mental, not physical or stat-based. Figure out what the characters' (and players'!) levers are. RPGs aren't just tabletop war-games with fewer combatants. They don't even have to involve fighting at all!
There're some obvious and simple levers: blackmail, extortion, bribery, hostages. But why make him so two dimensional?
Masterminding before the final battle
In his ideal world, the players will be questing to save him, not defeat him. Perhaps they'll believe he's possessed by an evil spirit that can only be banished by bringing him the Amulet of Ridonkulous Power to "give him the strength to throw off the evil spirit". White-knight syndrome is a definite weakness.
But even without willingly and consciously working towards his ends, they can still be made to do so, because of their weaknesses. Perhaps they won't suspect that the little orphan girl the party found and basically adopted and spends all their time protecting is his avatar or minion. Perhaps they don't know about Nystul's Magic Aura, and happily treat magic items they get as whatever they identify as, not realizing that they might also have additional secret enchantments. Perhaps every side-quest and sub-quest they have been on to get to him was actually him getting them to do his dirty work for him. Perhaps that town of goblins they wiped out was really a town of polymorphed humans. Perhaps that evil wizard they slew was just a disgruntled ex-employee of Mastermind. Perhaps they think a dungeon he created is a good and safe place to go delving, even if there's a nearby river that could be diverted into it once they're inside. Perhaps they were the ones who diverted it, for good reasons, not realizing who was in that dungeon...
Rather than give them a simple, two-dimensional foe that they can fight with on their own combative terms, have him outclass them through better intel, better strategy, better tactics, better execution, better PR, and vastly better logistics than they have the resources to manage (see sites like: https://www.themonstersknow.com/, or Tallow's Deep from Dungeon magazine issue #18, for intelligent foes).
Ultimately, if he is always well-informed about what they are doing, and what they are doing is always what he wants him to be doing, then why would he ever WANT to go toe to toe with them? They're his minions, they just don't know it.
As much as possible, make everything their fault. He's better informed? That's because they told that stuff to one of his spies. He's better equipped? Well, they took on a job as caravan guards, and literally defended his weapons from bandits. Didn't they ever look in the crates they were protecting? His strategy is better? That's not for lack of vague foreshadowing that they will facepalm and think they should have taken as hints once they look back in hindsight.
Most players are used to simple Monty-haul dungeon-bashes. That conditions them into playing as gullible, naive simpletons with a white-knight complex. Leading them around by the nose to have them end up performing atrocities in his name should be child's play for any thoughtful and well-funded villain, and makes the final reveal so much sweeter and more poignantly memorable.
Money is basically mana
You can use money to perform the kind of world-changing "magic" there isn't even spells for. So use the money to change reality, not just to buy gizmos. One way is to literally pay to have those changes made. An easier way is to just buy the papers, town criers, bards, etc. That's all it takes to change that neighboring town in everyone's eyes from "noble neighbor allies" to "brutish barbarian invaders". So how the players see the world will ultimately be how he wants people to see it, filtered through his spin. You can pay any number of mooks and thugs to do your bidding. For anyone the characters interact with, ask yourself "what would this character do if he was in Mastermind's pocket? What would Mastermind need to do to control this person? Money? Extortion? Build an orphanage?"
[2024 edit: found a really fantastic furry webcomic, http://zerosincomic.com which explores this theme of "bad guy controlling people through the superpower of money": strongly recommend reading, and not just as a really well done exemplar of this "rich-baddie" trope.]
Mastermind doesn't necessarily even need to come across as bad or evil to most people. He could be building zillions of orphanages (great place to get loyal minions from). Providing schooling for everyone (good way to get the masses indoctrinated into his cause). Yes, they'll argue, he has slaves and work camps and dungeons with torturers and executioners, but so do all kings, and only bad people need worry about that anyway. And the work he has them doing is for the benefit of society! Yes, his onerous taxes are making life tough, but have you heard the news? Better to be poor but protected by a strong army, than end up dead or worse! Yes, he pays necromancers to build him an undead army, but every army needs cannon fodder, and isn't that so much better than asking the living to take that role?
Bringing Them Low
But eventually, yes, they must find him out and move to confront him... in which case, what if everyone has become convinced the party are the bad guys? Suddenly their assets can't be called on, and they are wanted by the law. They have to make the moral choice between fighting their way through innocent civilians who've believed the lies about them, or finding another way. The playing field is leveled a little.
Meanwhile, for Mastermind, money is a summoning spell, essentially. Will he summon better armor or weapons? An Iron Man suit? Sure, but he won't use them. Why not? Because, just like a summoner, he can also summon someone who knows how to fight, to wear and wield such equipment better than he ever could have. And he can summon any number of such people, and any amount of equipment. A zillion Kronks and a zillion levers, or whatever. Masterminds aren't there to grandstand to foes; that's the job of heroes and super-beings and major henchmen.
Instead, any Mastermind's goal is to try to win, to complete their grand plan.
And once, finally, the party has made it past all his paid minions... well, then it's the same as when the party attacks a powerful summoner and defeats their summons: he's alone, weak, and begging for his life, his plan not even half-complete.
Except a smart villain won't beg unless he's confident that's what the party will react best to. Would the party be more likely to let him off with a line like "I know, I deserve to be killed, and it'll be a relief if you do, but please - I did it all because my daughter is being held hostage on the demon plane of Quartz Butte, please save them even if you slay me... I have a magic item conveniently right by my throne that will help you get there, that I had totally just recently acquired for my rescue mission, and not to banish anyone who attacks me to the demon plane or anything. I'd go with you, but I was hurt in your attack..."
If it's OK for the players to turn the tables and wrest a victory from certain defeat, then so can Mastermind.
And however it's done, it's important for the Mastermind to bring the party so low, that they can only escape by digging through the bottom of the barrel, before they defeat him. Without him at least demonstrating that he's a viable opponent, able to easily destroy them, a victory against him is no more significant than against any other commoner or goblin. Whether true or not, they must believe that a TPK was definitely on the table.
The Final Confrontation
"NO!" you have perhaps been crying for hundreds of words by now. "I want a climactic set-piece battle!"
Then have one, but not with the Mastermind. Perhaps, as in the James Bond series, with the colorful lead henchman. With Jaws or Odd-Job, not with Drax or Stromberg. A Mastermind should flee any fight, leaving grunts in his path to prevent pursuit.
Instead, the Mastermind's ultimate fate can -- I'd argue ideally should -- be something the players bring about, preferably causing him to screw himself due to his evil plans or hubris, rather than fighting directly. Lead the players into replacing the ashes for his final summoning ritual with gunpowder, blasting apart his protection circle so the thing he was summoning kills him, but then the payers have to take it down in a real climactic battle; or place his own nuke into his escape capsule, and the final battle is actually a "get to minimum safe distance" race; or...
A Mastermind should hoist himself on his own petard, not duke it out with the party like a grunt.
Of course, this often also gives you the chance to bring him back as a nemesis. "Well, did you actually see his HP drop below zero?" But don't bank on that: many players are wise to such tricks, and will make sure to make sure!
GMing a Mastermind towards narrative story beats
If you're like me, you're likely not a genius. So GMing one is hard.
Fortunately, while GMing looks like bookkeeping/accountancy, for many(*) it is first and foremost performance art, and nothing behind the GM screen is set in stone. No matter how long the players sleep in at the inn that morning, they will never arrive too late to rescue the damsel or disarm the bomb.
Like a magician's gimmicks, this is all kept secret: every GM claims to adhere strictly to the tale. There is nothing up their sleeves.
That's because, even though the story follows the players' whim, rather than the other way round, it's still very easy for players to feel railroaded in a dynamically-crafted narrative, especially whenever they hit story beats that were planned long in advance. Railroading feels bad because RPGs are first and foremost meant to be tales of choice, and people want their choices to have consequences. So it's often difficult to make a planned story beat not feel clearly designed and worked towards, and savvy players will understand that it was you, the GM and not the Mastermind, that was working towards that story beat.
But there are two Great Mythic Narratives of GMing (based on irrefutable, bedrock-solid truth of course, like all great myths!), which allow players the freedom to feel as if they are not being railroaded.
The first of these is the narrative of "foolish players screwed themselves out of the easy path". Players (or their characters) learning just too late the "easy" ways they COULD have done it. Whaaaat? If we'd got the hints and woke at 7am, we could've just rode into the Evil Castle on the breakfast wagon from the inn, skipping the entire Labyrinth of Doom? This is anti-railroading: pointing them to where they missed the tracks, too late for them to jump back on. The tracks don't need necessarily even to have existed (perhaps if they'd woken at 7, the bridge would've been out, getting them past only the first level of the Labyrinth; or perhaps the wagon would have left at 6) - but so long as it feels like a "D'oh, we should have known!" rather than a "gotcha!", the Foolish Players narrative can be played to.
This first narrative is part of why, above, you made everything as much as possible be the players' fault. The players will know (incorrectly) that they always had the option NOT to choose to screw themselves. It's easy for us to blame our GM when NPC actions screw the PCs; but as players we will tend credit ourselves, if our PCs' actions benefited or screwed us. We take ownership for our own actions, and so it feels as if our actions had consequence.
The Second Great Mythic Narrative of GMing is the narrative of "clever players frustratingly doing the unexpected and derailing the plot". Players will be cunning about outwitting characters, but they will often be far more cunning if they feel they might outwit the GM.
Done well, we players can feel smug satisfaction at our sneakiness in dealing with the villain the easy but narratively interesting way, screwing our GM out of a climactic fight we imagine you'd "doubtless been building up to for months", even if in truth you felt that if it really had come down to a fight, you'd have failed us as a GM.
So as the players find cunning and amusing ways to screw your Mastermind, feel free to visibly pull your hair out about all of it, publicly wailing and gnashing your teeth, even though that's exactly what you hoped.
Never let us players know if an adventure went according to plan: we all appreciate a GM who can adapt to the unexpected and roll with the punches the players send. We simply have no need to know which if any of those punches were finely choreographed.
(* - for others, GMing is a sacred trust, to be carried out with absolute and inviolable respect for the truth and integrity of whatever's hidden behind the GM screen. There's often very visceral and emotive disagreement between these two storytelling styles. If you fall into this latter camp, much of this answer may not be for you, and may even be anathema: my apologies if so! A stats-based-balancing approach may be a better fit in that case.)
