Depends on the promoter, managers, trainers, and whoever else you have to pay. If you're signed to Don King promotions, you end up in the negative though, I know that much.
An average top level fighter. How big is his cut, after the manager/promoter has taken his?
I know for the Pavlik v Taylor fight......Pavlik got 1M and after everyone was paid he went home with about $660K (roughly).