What is the theme of his party? Oriental Trading Company has a lot of goody bag favors that are cheap. Kids mostly just want candy anyway.
You can go to Michaels or Walmart and purchase plain little paper bags (sort of look like miniture grocery bags) use a sticker that is the theme of the party to decorate. Put candy in the bag as the treat.
Or I always hit the "dollar" section at Target as they are clearancing out their items. I am always able to get goodies there for 25 cents a piece.
Goody bags are my speciality and I have always handed them out at my kids parties.
You can also get miniature candy bar wrappers printed off of ebay for only a few dollars a sheet, wrap those around those little chocolate bars and put those in a little baggie. You can also get labels for Hershey Kisses made.