I have no idea on cost down here (I'm from Indiana) but don't skimp on the sub-base. If the ground below settles, so will the concrete.
In my day job, I work for a construction materials testing company - so I've tested a lot of concrete. I'd suggest 4000psi instead of 3000 as listed above. Most slabs, driveways, sidewalks, etc are 4k. 3k is typical for foundations, however. If the barn is not going to be heated you will want air-entrained concrete as well. The air voids within the concrete will allow for freeze/thaw and you'll end up with a better product of the life of the slab. Concrete WILL crack, we just try and minimize how much, and direct it where.