I'm thinking about building a 28' x 30' detached garage to store an extra car and act as a shop. I called the concrete guys that did my house - they want $4,200 to pour the floor. It will be a monolithic slab (floating slab) so its about 4" thick throughout and then near the perimeter is tapers down into a footing about 12" thick. Does that sound right? What is the going price for finished concrete? What if I pour it myself - I've got a few buddies I could rope into a day of work? It might end up 10 x 10 before I can afford it .