I wonder whether Hamas faces a substantial Guns vs. butter trade-off
According to MEMRI, probably not a significant trade-off. MEMRI says life in Gaza was decent under Hamas, despite the restrictions that Israel imposed on the strip as a result of their activities.
Before the present war, the Gaza Strip – which has not been under Israeli occupation for 18 years, since the Israeli forces withdrew from it in 2005 – resembled a local Dubai more than it resembled the poverty-stricken city that is envisioned by many.
MEMRI is pro-Israeli organization according to Wikipedia.
Just to throw a more concrete figure here on the title (and 1st para) question:
Estimates of its annual military budget range from $100 million to $350 million, according to Israeli and Palestinian sources.
Also, like in Hitler's Germany or in Russia today, the large military expenditure does actually employ people, who apparently got paid, even by Hamas. And then the workers spend that money locally. ToI wrote in 2016 that
$40 million of the annual total goes to terror group’s tunnel digging work, which employs 1,500 Palestinians. [...] The average salary for an excavator is $250-$400 a month — relatively high for the Strip, where unemployment is rampant. [...]
By way of comparison, the budget of the last Hamas government, which dissolved in April 2014, was $530 million. In other words, some 20 percent of the budget was funneled toward arming the group with advanced weapons, digging tunnels, training, and salaries for Hamas fighters.
For comparison, Russia's military expenditure was estimated to be around 14%-16.5% of their budget for 2019–21, and 17.7% in 2022. Current estimates are more difficult because Russia started to publish less data, but that source estimates it's now "37.3 percent of all budget spending, 2.5 times the pre-war average".
Also, there's something not discussed in sources above about Hamas' budget, namely how much military aid Hamas got. I.e., their estimated military budget appears to assume Hamas paid for all the weapons, which is probably more true for the locally produced ones, but less clear for smuggled import parts, especially those of Iranian origin. Hamas themselves have claimed that Iran donated them the equivalent of $70 million in arms and parts, but the time frame is a bit unclear.
Also, Iran might have been less willing to fund social projects in Gaza, instead of arms. And Israel's COGAT less willing to let go through any openly Iranian projects in Gaza, even peaceful ones. So, assuming that all that was a straight trade-off ("guns or butter") requires more complex alternate history assumptions, such as world in which Iran and Israel are not still fighting a shadow war.
I do note that you've phrased that at present tense, but that's probably much harder to answer. What's Gaza's GDP right now, for instance?