Purchase a small fishbowl and put a few goldfish in it. Leave it in his bedroom, or in his classroom if allowed, and attach a note that reads "Out of all the fish in the sea, I want you to go to the dance with me." Goldfish are less than a quarter each, so this is an inexpensive but very creative way to ask a boy to a dance.
You'll have to get permission from his parents to do this one. Fill his room with blue balloons and a poster that reads, "Will you go to the dance with me? If yes, pop a blue balloon and send it to me. If no, pop a yellow balloon and send it to me." Obviously there won't be any yellow balloons so he can't say no! This again is a very inexpensive way to ask.
If you have a little more money to spend, look into purchasing a billboard for a day and use this to ask him to the dance. Find one that is on his route to school or near his house. Let his mom or friends in on the surprise, that way if he doesn't see it they can slyly point it out.
Buy a cheap puzzle at the store and put the pieces together. It should be a puzzle with very few pieces. Then once it is together paint the puzzle a solid color and paint "dance?" on it. Then take the pieces apart and put them back into the box and leave it for him to find and put together.