I'm pretty sure the cheapest 2.4ghz receivers are the Orange models which are compatible with Spektrum DSM2 kit. Well under a fiver for a six channel receiver.
I use Spektrum and Corona equipment, the latter is used on retrofitted radio sets. The Corona receivers are about twice the price, but come in 4 and 8 channel versions. Still cheap at well under a tenner. I guess the price differential is only likely to be an issue of you have a ton of models to fit out.