I would doubt that it is completely true if a boxer is able to secure a high level MMA contract. 3-500k+ w/PPV %'s + sponsorships. With that most boxers could make more in MMA. Toney's boxing promoter even said that the UFC will be good payday for Toney, so it's worth him doing. That says a lot, if there was a huge discrepancy in pay Toney most likely wouldn't be doing this. The only guys getting huge pay days are the Floyd's and Pacquiao's of boxing. I'd be shocked if guys like Paul Williams, Chad Dawson, Kelly Pavlik, Cintron, Berto, etc etc are earning more than say Rampage or Shogun. No way. Either way it's really not about the money for most of these guys, it's about passion. If you're at that level in boxing, you've probably been committed to it since you were a kid, so it's going to be hard to pry these guys away from a sport that they love, even if they could get a few extra million.
Now, if you present MMA as a legit opportunity to showcase their boxing skills and prove that they're the best fighter in the world, show them some respect, maybe you interest a few guys here and there.
Those guys are still making really good money, Toney cant anymore that is why he is doing this fight, it was reported he will make between 750k and 1 million
I guy like Cintron could make the transition, he wrestled in high school and has more of a power puncher style, I think he still makes good money though so he wont try it until the well runs out in boxing and than it will be to late for him to make an impact