There are a few SPAD 2610 (RCCA 1/12 scale rules) legal designs out there.

However, I would generaly disagree with Combat Addict's definition. Most combat planes do cost more than $20 when bought as a kit.

You can scratch build for a lot less than a kit would cost.

Some of the least expensive and most durable scale kits come from JK Aerotech. Jay Lawless makes a small selection of kits as well that are inexpensive. And there are a several others that aren't coming to mind right away.