G'day, I built my Studio for $20K and as Mod Bod said thats just materials only. I started with 9 x 12 room on a 6 inch concrete slab. After insualting the room with "Sound Screen" (Rock Wool) I covered it with 2 layers of dry wall to get to about 1' thick. I then built another frame inside that room room completely free standing. The resaon for this was to not only reduce noise but to isolate vibration. I insulated these walls with Rock wool then lined with another 2 sheets of dry wall.
The Observation window was built inplace. 8mm Glass at 90 degree for the Mixing side and another sheet a 5 degrees on the studio side.
I did think of ventilating the room but due to the cost and complexity (to ensure no noise) I opted not to do it. I have to say since the room is so insulated it does not get hot or cold unless I indruce heat or cool. In saying that it does get stuffy. If I have a drummer going nuts in there it does get warm.
Happy to to send pics or offer any other advice.
Mark