Yeah, it's really expensive to "soundproof" a room. It's actually pretty near impossible, even in good studios you can usually hear the drums coming through the walls. You can deaden the sound alot so it isn't so annoying to the neighbors or others in the house. My basement studio isn't bad, yes, it is audible outside, but not totally annoying. The room is basically a box, built on a frame off the floor and the ceiling doesn't touch the floor joists above. The walls are framed with 2X4s top and bottom and 2X3s staggered to mount the sheetrock to, so the outside sheets and the inside sheets are not mounted to the same piece of wood, the window has a piece of 4" foam in it, and I have hung some rugs and studio foam on the walls. The rugs are mounted with 1" foam between them and the wall, if they were hung touching the wall they wouldn't do any good other than killing some of the highs. The doors are exterior type french doors, so they have double glass, single glass wouldn't help. It's some work, and yes, it is still audible in the house, but it works well enough. Oh yeah, to make matters worse, my basement has a floating wood floor instead of concrete (which would have made it much easier), they have to do that where I live because the ground swells and contracts so much it would destroy a flat concrete floor. The drums are also mounted on a small riser which is on rubber isolators (the big black rubber stoppers you can get at Lowes).
Perfect, far from it, useable, yes, for now, I still need to get around to finishing the rest of the basment though.
The other thing is that it is so sealed, well there is one duct into the room (which also lets sound travel out), that it can get really hot pretty quick with a couple of amps and bodies in it.
<$0.02,
Roger