Sublimelobc,
It's hard to give a ballpark to be honest because my father in law had a lot of raw material for the framing, etc. Then, I didn't have to pay for labor, etc. So, I got it pretty "cheap" actually. The studio is only 10' wide, 17" (13' to the vocal booth) long and 9'+ high.
DK,
Yes, the sidewalls are parallel, unfortunately that was one of the constraints of building the studio in a 3 car garage with 2 cars.

However, the ceiling above the listening position is slanted, that was possible since the roof of the garage is a seperate roof from the house. I do have Microtraps on the sides though for 1st reflection points, and the speakers are placed symmetrically. I will also mount a Microtrap on the ceiling and another Minitrap HF on the vocal booth wall for 2nd order reflections. This weekend I will actually measure the room, since I watched the webinar on speaker optimization on mixonline and man was that depressing! I have done a mix since the studio is done and it translated pretty well. I do have an issue at 130-160Hz though from running a test audio CD from Realtraps. Btw, if anyone is looking for a cheaper solution to Realtraps, I just found these guys and they seem to be really cheap,
www.gikacoustics.com.
For the space I had I am pretty happy, plus we now have an extra room in the house again.
Ken