There is no number for exactly that, but there are a few options on CRB p293-4
- Formulas show the cost to learn how to make such an item
- Hirelings are level 0 people that can be hired for 1sp (+0) or Skilled for 5sp (+4) per day
- Obviously, this won't work for most items, but gives some idea of a place to start
- Spellcasing Services gives a better idea of what a spellcaster's time (and spell slots) are worth
- Note the modifiers in the text as well, 100%+ extra for Uncommon spells and 25% more for spells that take a significant amount of time to cast
Regardless of the actual costs you settle on, it should
- Be adjusted by the level of the item, to keep with the economic balance Paizo established
- Be pretty negligible in cost compared to permanent and consumable Equipment
With these numbers in mind, my suggestion is just to use the Formulas cost for permanent equipment or 10% of that for consumables unless you want the identification to be a dynamic plot point because I wouldn't consider every item interesting enough to invest more effort into.
For that price, you could discuss with your group whether they think it's more interesting and fair to either roll 1-3 times with level-appropriate NPC checks as though the NPC's had Quick Identification and Assured Identification (my first thought), give them the 'success' results, or whether you should roll but not charge them for failures. I haven't tested this in-game, but the numbers are easy to work with, small enough that it won't bother most players, and large enough to cause the party to consider investing in the skills themselves.
My personal suggestion to a group I'd be in, for the sake of realism, would be paying the Formula cost on a successful NPC roll; the larger/higher level the settlement, the more NPC's the party could find to attempt the roll. If it became clear that the party was going to use this as their main efforts of determining items' abilities, I would also suggest there be a level of item for each settlement that results in auto-success (for the Formula price).