I think it Varies on what you are able to spend. I have 3 boys and they all three have been invited to one persons birthday party before...I usually call the mom of the birthday person and ask what are some of the things they like. I can say this most kids get plenty of toys summer is coming so if it was me (and I have done this with a big hit at the party) I have bought a beach towel of their fav color or theme on it, superhero with a bag filled with summer fun items...bubbles, goggles, pool toys or any outside toy or a bucket filled with pool items and a towel and pool shoes. Now if these folks don't do the kiddie pool thing and don't want that then I go for a blanket for a kid same as above fav color superhero etc.. with some cool things for their bedroom ex: nightlight, pillows, some educational music on a cd or a learning dvd they might like. Oh, big hit kids always love the crayola color wonders with any of those items. I having three kids invited spend around 20-30 dollars...but it's up to you...
I have had parties and spent a good amt on them but a party is just that had folks bring 3 kids and bring one gift for under 10 dollars...didn't matter the thought was there and my kids really just wanted to enjoy their company of their friends...it doesn't matter what you spend it's up to you I've got things at the dollar store and know one was any more wise they just loved it.
Best of luck on what you decide have fun at the party.