and I'm not talking about how foolish one would believe BYU would give up money to play Utah at home.
I'm talking about the idea that BYU would do the same if they joined the P12 or B12, as you say Utah would have to do to BYU.
BYU and Utah's interests are not the same. Though they are similar, their underlying motivation (ie: Money for Utah, Mission work for BYU) is significantly different. IF the scenario you pose above is the real reason why Utah may drop BYU, it would not be recipricated if the situations were different (as you suggested). BYU wouldn't drop a football game with a beloved rival because they may lose out on a few $100k.
The difference in amount Utah would make playing in Provo, in fact, I suggest playing BYU in Provo pays out more for Utah, then playing in SLC. Why, Utah is under a equal share plan in the PAC 12 so it matters not many home games they play as it would if it wasn't equal share. In addition, if I remember correctly, LES is 20k+ bigger than Utah's stadium. The ticket revenue and concessions sales payout, plus additional tv revenue from a possible ESPN game would all dwarf Utah's revenue from tickets and conessions.
But back to my main point, BYU wouldn't drop Utah, assuming this was the only reason, for a few extra $100k.