Sussex models is selling the Planet 5 2.4Ghz Transmitter & Receiver Set for £36 which they describe as:
Advanced 2.4GHz 5-channel professional transmitter and receiver
Includes
2.4GHz 5-channel professional transmitter
2.4GHz micro 6-channel receiver

I know nothing about this gear, just saw it on their site, but for £36 it has to be the cheapest 2.4 GHz control you are going to get.