BYU gave Notre Dame an out in the contract to play in Provo - and Notre Dame took it.
I don't know the particulars of the contract to play in Vegas - but I'd be shocked if BYU is getting less than they would have in home game ticket sales.
AND - BYU still has 6 home games, so it's hard to say they are giving up the money from a home game when they're still getting a home game - just against someone else. The replacement team doesn't have Notre Dame's prestige, so maybe it doesn't sell out - but they'll still sell some tickets.