I just quoted a job just like this last month, so here we go.......
Concrete is going to make up half that price, easily. My go to guy for concrete is charging about $8.00/ft to place pour and finish, including concrete and rebar, as well as a load of gravel. That works out to $9600 for your slab. This is for a standard 4" garage pad.
Considering that pads bigger than 26x26 should usually have some piles underneath them, as well as perhaps a thicker slab for the heavier equipment you are prone to parking in bigger builings (tractors, etc), you can add some concrete into the job and expect the bill to rise on that at least a grand, if not more.
Then you have to buy the materials for this job.... Lumber, shingles and siding. I had my local supplier quote me their absolute best price on a 30x40 shop with 12' walls just last month and it came in at $7600. I am pretty sure that was at or near their cost, as they managed to come down $2000 from their original quote, which I had a contractors discount on.
This already has you at about $18,000, not including the contractors labour for framing, siding or shingling, and not including your overhead door, which will be around $2000 if you get a decent quality unit.
So if you want this building to be built for under $20000, you might want to look into a gravel floored pole shed, of find a contractor who just works for fun.
