Well, you might be able to get a used plane and engine for less than 100 bucks, but then you need the radio and the field gear. Just keep on saving. Don't buy something cheap because it probably wont last.

Occasionally, I have run across a complete ready to fly .40 size trainer (with radio and engine) for sale used right at $100. Usually the planes this cheap are n need of repairs, and if the radio is useable (on a currently legal frequency, and meets the "narrow band" requirement) the NiCds are usually no good.

RIGHT NOW, I am getting ready (doing repairs and testing) to sell off a USEABLE trainer, complete package deal, and I expect to sell it for about $175. That's about what you should expect to pay for a GOOD, flyable used aircraft with radio and engine, not needing replacement batteries, or repairs for safety... just fuel it and fly.