These exhibition fighters coming from MMA are getting paid more because they have built their name in MMA and they have already gained a legion of fans they follow their idol in boxing matches and people out of curiosity are paying to watch these matches, and so far the fans are not disappointed because its action-packed like what happens on all of the brother Paul's matches, there are rumors that Jake Paul will fight Andersson Silva this match if the deal is made is going to be another blockbuster in the making.
You're correct, I'm not really following MMA, but if there's an exhibition fight for MMA he will also get a huge paid too since many people are want to see their idol fight, so it's not only on boxing. Maybe boxing is bigger than MMA and mostly exhibition fight happen for boxing, not MMA.
Personally I'm not fan of exhibition fight since it's a fixed match, but sometime I will bet if the odds is worthy for me. Just like the cancelled event where Jake Paul will fight against Tommy Fury, Jake Paul is the underdog and it's make sense to bet him since he's a clever businessman.