You can build one in an afternoon if you stick with it and it can be made for under $100. We didn't stain ours because we like the look of the bare wood (haha) and it wasn't worth the effort.
After we had the idea, we started looking around for plans. None of them really fit our situation exactly, but we used some ideas from each of them. You can manipulate, add or subtract from these plans. They are just what suited us.
There is a Google sketchup file I uploaded that is the project.
Step 1: Materials
~Drill with various bits
~Saw - capable of cutting at right angles (miter saw)
~Clamps - preferably C clamps.
~Wrench - preferably ratcheting
~ (1) Twin sized mattress
~ (1) sheet of 3/4" plywood (get it cut to 39 inches by 75 inches at the place if possible)
~ (12) 2x6's
~ (6) 2x4's
~ (16) 3/8" carriage bolts
~ (16) 3/8" ID 1" OD Washers
~ (16) 3/8" nuts (locknuts if you wish)
~ (1) pack of 3 1/2" screws