S.M.
I spend $12 on birthday gifts for classmates. For our very best friends, I might go up to $25. So I think you in the ballpark.
The organizer should see how many kids want to contribute and then offer an estimate of "donaiton."
Just a thought: Does the family want a trampoline? It is a lovely idea to splurge on a big gift... but I would not want one for my kids.