You can't do your own transmitters unless they're infra red for legal reasons, as you need to get it certified and it all gets very tricky. You can however buy transmitter modules fairly cheaply which are pre certified and you just need to attach a micro controller and joysticks to. Either that, or if you don't stick with DSM2, I have in the past found cheaper transmitters and receivers, but i couldn't find the link again..

I've hacked the servo boards in them before for Nanoweights; just switch the potentiometer out for a pair of ~2.2k resistors. Converting the rest of it should be fairly straight forward; remove the potentiometer from the casing and cut out the little bit of plastic that stops it rotating the whole way round.

Motors are not worth building yourself; the one's I linked are very good, and at £2 each they're not going to get much cheaper without going to Nanoweight components.

And if you're going cheap then China is your friend; very low prices, and nearly always free postage. Takes 2 weeks but that's usually ok. I buy nearly all my parts direct from china.