True value is increased in one of three ways:
1) Getting more benefit for the same or less cost
2) Getting the same benefit for less cost
3) Getting marginally more benefit for marginally less cost
So the fact that BYU has a ESPN contract is a moot point in the argument. The assumption that the BYU contract is worth what ESPN is paying and that the current ESPN contract meets the cost/benefit expectations for both BYU and ESPN makes this point void when discussing a possible move to Big 12.
So when you remove the fact that BYU has a contract you would just at the current Big 12 contract and ask does adding BYU meet any of the mentioned three ways to save