They can make much more money on just tickets sales by holding the fight at Wembley. However a Vegas casino will give them millions of dollars to hold the fight in their casino rather than at Wembley, which will charge maybe a million to stage the fight there. But the main thing here is that HBO won't do a PPV fight featuring two non-American boxers from another country. If Manny and Ricky want the PPV money from America then they have to hold it in the States.

100000 tickets at £100 average is ten million quid, or 15 million American.
Let's say 13 after fees. A Vegas casino will pay five million for this fight, so let's say they're ahead eight million by holding it at Wembley. Let's say it does only 500000 PPV buys in the States. They get roughly half the total PPV take, so they'd make $ twelve and a half million from the PPV. Even if it does Pavlik-Hopkins/Jones-Calzaghe numbers they probably come out ahead, but they're guaranteed to do better and the upside is massive. If it does a million PPV buys they split $25 million just from US TV. They're not having it in Vegas because they'd make more money holding it in England.