If you're patient and diligent in your search for cheap stuff, you might find a servicable radio system for well under $100. you can get a new .42 engine for about $50. If you get into this hobby though, you'll end up $pending and $pending and $pending, so even if you can get away with starting cheap, continuing cheap is next to impossible.

Cheap aeroplanes? You can make really cheap airplanes, some of which fly remarkably well, from Coroplast (corrugated plastic, think election signs/gas station ads) and other materials. For info go to:
SpadToTheBone

Or look at the T52 foamie trainer at:
JKAerotech