50 bucks does limit you pretty badly, but depending on how small you want to go, and how much experience you have, you can take those little micro RC cars (the really tiny ones, for like 5 bucks) and use the electronics in them for an RC plane. it usually needs some modifications to get it working right, but its the cheapest route to go.

if you didnt go with the bit cars, you're gonna be looking at more money. but, last time i was at the hobby shop, they had a GWS flight pack that had two GWS narrow servos, RX, ESC, and an NiMH pack for 60 dollars. you'll still need a transmitter, charger, motor, and prop (another 60-80 bucks, roughly). ebay can be hit or miss on the equipment you might get, but there's usually some stuff for sale there.

unfortunately, micro RC is still pretty new on the market, so prices are still high (micro RX's still cost 75 dollars and higher). give it enough time and they'll drop in price. in the mean time, you could spend 30 dollars on an AeroAce and use the electronics from it to build your own twin engine plane. myself and LOTs others have done some pretty cool ones with it. all it takes is some glue and balsa.

nick