Simple, Cheap, and Effective Hawaiian style spearfishing gun.
Step 1: Materials
2 ft Rubber Surgical grade Tubing or Exercise bands ($3-5)
Small Section of PVC 1/2in - 1in in Diameter ($2)
1 Youth Arrow with safety tip ($3)
4 - Zip ties (.50)
Total cost for these materials should be less than 10 bucks, if not free. All these things are pretty easy to find.