Kathmandu self-inflating mattress
Thought I'd throw in my two-penny's worth...
On my last big camping expedition in the Western Australian desert, I took a cheapie Kathmandu self-inflating mattress cos at the time I couldn't afford anything in the more expensive Thermarest range. Apart from the fact that it rolled up mega-small, it was REALLY comfy whether sleeping on deep sand or rock-hard salt pans. Coupled with an emptied wine bladder as a pillow (I'd forgotten to take a proper pillow!) and a Kathmandu down sleeping bag, I had some of the best night's sleep ever on it!
So if you're looking for a cheap but effective option, I heartily recommend the above!
PS: I had help emptying the wine bladder, in case anyone's wondering...!!