If that game is played in Provo, it would just count as one of the games under the ESPN contract and BYU would get paid whatever $$ is called for in that deal, and then they would have to pay Notre Dame a few hundred thousand dollars (my understanding is that with H/H deals, the home team typically pays the visiting team something to cover expenses)

If they play in Vegas, ABC/ESPN would almost certainly pick up that game for national broadcast in addition to the games at LES, and would pay an extra fee for the TV rights, and BYU could still host (and get paid for) six home games like they always do, plus getting a pretty good payday for the Vegas game.

Playing in NFL stadiums is a cool experience to show off to recruits, and it's fun for fans.

A game in Vegas would be at least 50/50 BYU fans to ND fans, and given the proximity to Utah, probably more like 60/40 in favor of BYU.