If you want to save as much money as possible you can just cover your studs with panda-poly, the white plastic some people use for a reflective material. If you do not need something substantial for strength you can close off your grow room using the same material you use for your reflective material. If you use the 6 mill. thickness panda-poly it will block light so you will not have light leak problems. The black side blocks light and the white side reflects light. You can even make the 'door' for the room by hanging a piece of panda-poly with Velcro on the edges and on the edges of the 'door' frame and seal it that way when it needs to be closed. A couple bungee cords on the ceiling above the 'door' frame will let you roll up the 'door' panel and hold it up with bungee cords.
I am not saying it is the very best that can be done but if someone has a smaller budget or just wants to save as much money as possible it is a viable option.