I have heard of a "scratch slab", which is a simple, rough concrete slab to cover the dirt in a crawl. Maybe that is what he is referring to as a rats slab? If done right a slab can help with moisture situation and just make everything a heck of a lot neater, etc. I can't even begin to understand how any kind of slab for a small building could cost so much though. I would guess that the entire foundation shouldn't cost more than a couple thousand dollars.
Actually, for a small building or addition I would see how much you can save by putting it on poured piers instead of an entire foundation. Heck you could dig six 2' wide by 4' deep holes, put in the form tubes and pour your own footings in an afternoon and save $11,000 plus the cost of a foundation.