Since it will be spring, you could make Mother's Day Cards. You can buy a package of colored paper (Cardstock) at JoAnn's or Michaels or Hobby lobby. Usually a pack of 50 sheets is about $4 (or less, on sale). Each sheet can be cut in half and folded, so one whole pack would make 100 cards. You can buy plain white "invitation" envelopes at some Wal-Marts and I know you can get them at office stores. Also, Jo-Anns and Michaels sells them too. Have your son cut out flowers or pretty pictures from Magazines, or just print some off the internet and glue them on the front. Then He can write "Happy Mother's Day" on the front or you can purchase a rubber stamp and stamp pad (again from Michaels, JoAnns or Hobby Lobby) to stamp on the front.
My guess on the costs:
$10 stamp and stamp pad
TOTAL=$20 enough for 100 cards which works out to 20 cents per card. You could sell the cards for 50 or 75 cents each
You could also make Father's Day cards.
These are good for kids since they don't normally know what to give their parents on their own. and, don't forget, it will be the end of the school year soon. You could make some "You're a Great teacher" cards or bookmarks.
You could also use the paper to make bookmarks for kids that could say Anything! Buy some Disney stickers and decorate them.
Just some ideas. My son's school does a business fair every year for 3rd graders.