I used to think that's a lot. But when you add it up, that's about 12M for the whole 18 years.
According to google, in his current contract he's making 48M per year.
I'm not saying 55k monthly is not a lot (it is). But (in theory) he's paying 12M to financially support his children from 0 to 18 which seems somewhat fair. Now if the argument is that the mom is using the money for her personal stuff, sure i think that's wrong. But sometimes it becomes tricky to determine (house benefits the whole family, cars, etc). That's all part of the game. His best bet would have been to file for joint custody i guess...