Unfortunately my DD isn't Invited to school "parties" as the parents know shes diabetic and don't want the "extra work" that comes with it.
Well that is just plain wrong from those parents, I have a niece who is allergic to everything, her friends parents know and my Sister Inlaw just provides the food she can eat and it all works out.

Usually around $20 here for the kids school friends.

I tend to buy what I think my kids would like or what I would have liked. It really depends on what is 'in' at the time as to what I get. I also ask my DS's if they know what the birthday child is into and go from there.