IIRC, the NYTimes did an article on this. Basically, Mayweather has a unique business operation.

He puts up his own money to pay the costs of the fight, including his opponents purse. If you fight him, you are essentially working for him. In return, he gets a cut of the revenue generated during the event: T-shirts, food, broadcasting rights, etc.

I'm not sure if that cut is part of the purse or if this is the arraignment he will have with Pac, but the point is he has to pay expenses out of his purse. Apparently, it works out rather well for him...as he has a revenue stream coming in for years.

Against Mosely, he refused to pay the sanctioning bodies, so it was a non-championship fight. Obviously, he realized a big name boxer does not necessarily need a belt to be considered the champ. So why pay for it?