personally i dont look at it in those terms
it's all about the next fight. right now manny pacquiao and floyd mayweather are the cash cows in boxing. cash in king in boxing or anywhere else. they weren't when they started boxing, and they're not going to be at their current paygrade forever. pacquiao's pay scale has almost certainly peaked, and floyd's will be soon to follow.
eventually, barring retirement, they'll be fed to younger fighters to pass the torch and increase the earning potential of the next generation. if they stick around long enough these guys who are making big dollars will become roy jones, wandering europe and getting the trash kicked out of them for short money.