Coroplast: $10
10' gutter pipe: $6
Cutting board for firewall/engine mounts: $6
Bits and pieces (screws, fuel tank, wheels, etc...): $20

The coro and pipe will five about 4 or 5 planes, the cutting board, about 20 firewalls, and you can re-use the tank/wheels/radio/engine until you feel they need to be replaced. In other words, each plane is only about $5 after the initial investment.