That is extortionate!
I have never done the whole class party, we usually let them have a couple of friends from school, plus the usual group of friends they've had since being born (about 4 others) at home for party tea and cake.
Now they are older they can choose 1 or 2 friends to go to a local theme park with, the entry for which is usually about £15 each.
There is no way I would pay that. I would definitely say no.