In the Mayweather vs McGregor fight, one earned $275M and one $85M.
Often in UFC, fighters get a ridiculous amount of money for agreeing to fight another strong fighter.
Now about T&F and marathons, it is clear that Bekele and Kipchoge are in the much stronger position here than race organizers. How much do you think they should charge if a major marathon organizer decides to invite them both? They could just avoid each other, win their respective marathons and make good money that way. So the appearance fee has to be significant, especially since only one of them can win and get the 1st place money which they would probably easily get if they would avoid each other.