How much does it cost to add a room above a garage?

This would largely depend on the size of the room and what you want to do with it. In a very basic format consider the following:

1 day: subfloor

2 days: framing and insulation

1 - 2 days: electrical rough-in

4 days: install drywall and finish to paint-grade

2 days: painting and trim work

All of this assumes that you have done this work before. If not, add an extra day or two to each or hire sub-contractors to do the work. You must get a qualified electrician or your insurance will be null in the event of a fire. Building permit is always a good plan but often sidestepped in a simple re-fit. If you are in a strata-run (townhouse) complex, check your by-laws prior to construction.