Have you considered pouring the slab yourself?

It doesn't take much to frame up a shed slab and most tool rental centers actually sell different qualities of concrete (including the fiber reinforced stuff) and rent the concrete working tools for a portion of what you'd pay someone else to do the work (they even rent concrete mixers).

If you go this route, just remember not to overwork the concrete. Once it starts setting up, leave it be!

Just a thought.