the reason they cost more than a hammock (and also the tarps more than the hammock, if you're buying those separate) is the cost of the materials (good quality stuff, light stuff can be spendy) and for down, the man hours required. Check with anyone who has diy'd a down quilt, and they'll say that it wasn't that much of a cost saving over buying one, if you factor in the time.
It's a funny thing that a lot of people have, saying "wow, that costs more than a hammock!" but remember that the hammock is just a piece of fabric with some suspension and maybe a bug net. you can make one yourself for 10$ for materials and maybe an hour. But it's the equivalent of the ground sheet of a tent (way more comfy, but still just like a ground sheet) and those don't cost much, or do to much except give you a place to lie (that is either protected from the ground-seep by being waterproof or by suspending you above the ground).
The bug net/tent inner and tarp/fly and pad/under-insulation and sleeping-bag/top-insulation are what do the 'real work' of keeping you warm, dry and bug free, whether above ground or on the ground.