I built out my closet just put up some wooden supports and drywall. Floor was already carpeted, than I went to the Foam Factory store by my house and bought 144 1' by 1' 2inch think wedge foam, and spray adhesive and foamed the whole walls, than I bought a light and hung it up on the ceiling, and a fan to cool down the booth. Took me about six hours to build and cost around $450 total that includes foam, drywall, some 2x4's for support, a box of screws, spray adhesive, light, and fan.
here's a video of my setup. i've since finished the booth I had run out of foam and had to get more and recorded this before so.
YouTube - New Studio!
foambymail.com = cheapest place to get studio foam btw. Check em out.