"A xbee pro is approx 40$ and you'll need at least 2. Then add the arduino + wee 34$+20$ + hardware and you'll be at 140$ or more. It seems that you can buy 2.4Ghz small plane for that price. So I doubt that you can't find cheaper."
well i could make up my own boards from the atmega168 cutting a bit off the price there. and i can recycle other components.
it would be an intresting project too 
im doing rc on a shoestring here 
i have an indoor plane i built from packing foam, and it flies from the tests i done.
but the radio kit i have is a 3ch 35mhz set, making a 4+ channel set would be good
if i can find a cheap 2.4 ghz set for my planes then i will try that, but would like to look into other options 
edit: just noticed sparkfun has xbee starting form $22 (not inc postage)