Remember floyds pride is on the line, so he would probably cough up some of his own money for a rematch,
This is how I see it. Let's say Maidana won the first fight and got $1.5 million, in the rematch I would expect him to get $4-$5 million.
How much did Marquez get in all the rematches with Pacquiao? Considerably less then Pacquiao, the 5th fight didn't go through because they didn't want to pay JMM the 15/20mil he wanted. If they rematch Maidana should get a pay bump from the 1.5 he was getting but he probably tops out at 5/6 million.
Would probably depend on the boxer's fan base. I would imagine a guy like Cotto or Canelo would be looking at close to 50/50, whereas a guy who couldn't sell out their own living room like Lara would still get significantly less than Mayweather.