The thing to remember about hammocks, well, it's not actually about hammocks, it's about sleeping pads. Sleeping pads aren't there to make you comfy, they are there to keep you warm. The insulation underneath you in your sleeping pad is compressed to uselessness, so you need something to keep the ground from sucking the warmth out of you. In a hammock, it's exactly the same, except it's just air, not ground. Either way, you still need something beneath you to keep you warm.
You can buy a Grand Trunk Hammock for about 20 dollars, and a 10X10 piece of tyvek for about 20 dollars as well. As long as you have some ingenuity, you can rig up a pretty simple hammock set up from that. Just remember, that's for shelter, you still need a sleeping bag and pad for warmth.