No matter who you are, 25 million dollars plus a cut of the PPV revenue is a hell of a lot of money. It makes no sense to use what Mayweather earned in his last few boxing fights as the standard by which Mayweather should be payed for this fight. The reason is that Mayweather got that much money from fighting highly ranked boxers. Conor is not even a ranked boxer because he's not a boxer. McGregor is showing he's the bigger man by being willing to fight Mayweather under boxing rules.
If Mayweather were willing to fight Conor under MMA rules, then I agree he'd deserve to get paid more than what he's being currently offered for the risk he'd be taking but in this situation it's Conor who is taking the bigger risk. He's being brave enough to agree to the rules that Mayweather wants and with that, he takes a bg chance of being badly beaten.
If Mayweather were willing to fight Conor under MMA rules, then I agree he'd deserve to get paid more than what he's being currently offered for the risk he'd be taking but in this situation it's Conor who is taking the bigger risk. He's being brave enough to agree to the rules that Mayweather wants and with that, he takes a bg chance of being badly beaten.