While mine isn't exactly the same, I plan to build a 40x20 garage in my backyard. 25x20 will be the actual garage part, and 15x20 plus a small bathroom are planned to be used as a shop with the potential to make it into a mother-in-law studio. The bathroom is half in the garage, half in the studio. The estimate I got from one company was $25,000 for the framing, roofing, siding. Hopefully that gives you some idea. Those are northwest Oklahoma prices though, I'm not sure about your area. I felt like the estimate was pretty high, and I could knock a lot of that out myself.