Use a bike wheel for a bow (actually more of a slingshot, but you get the point)

The author uses everything off an old bike wheel to make a pretty knifty box.  Note that the eye bolts are not from the bike.  If I total this up (assuming free wheel) its about $5 US to build.

http://www.instructables.com/id/Bow-The-Amazing-Bike-Wheel-Bow/