i think that it depends on how well you know the children/parents. normally, i'd say $150 or so.
MATTEL has big sales every few months. They take cash only at the sales but you can get some steals. I went to the one before xmas and spent $680. I took home 3 HUGE bags of toys, including some Dora, Diego, Hot Wheels & Barbies (some of the barbie's go on sale for $199, regular price $299-349... I paid $100).
I have a "stash" that I keep in case of a birthday invite for my kids. It works great, so long as you have storage space.