I am obtaining quotes in NH to build a garage with a concrete floor. A two car garage 24x24 with no finish work on the inside. I have three quotes $20,000, $32,000 and $51,000.he size is 24 by 28 and the contractor just has to frame up the sides and 2 8 by 9 openings for the garage doors and then put 4 plain vynal standard windows and 2 entrance doors in and finish out with vynal siding.