The floors of your new space are a unique issue. Your garage is built with drainage in mind. Code requirements mandate that the concrete slab has a slope built into it to ensure water drains away from the home. The installation of the subfloor will correct the slope issue of your floor. Using a subfloor allows your new space to be converted back into a garage should the next home owner wish to do so.
A garage remodel is usually much cheaper than a home renovation because the foundation, roof, and walls are already built. Many garages also have electrical and plumbing installed to some extent. The actual cost of a garage remodeling project, however, depends on what the renovated space will be used for. While a new home gym won't require much in the way of amenities, converting a garage to an office, apartment, or bedroom requires adding significantly more creature comforts.