There is a lot more to this question:

Where are you located? The more remote, the more it will cost.

How accessible is it? If they can reach it with a pump truck it should cost less than having to wheelbarrow it in.

What do you plan on driving/parking on it? Thickness as well as psi strength will play in. Well built homes tend to go with 3000 psi concrete.

I would spit ball at least $5-$6 per sqft....

If you get bids make sure you get details on how much steel they plan on using as well.

Better get it poured now. Had a contractor tell me that concrete was going up $8/cu yd around March 1st. He.said the worst increase he had ever seen in his career was a $4 spike. Also rebar is up 15%, also lumber, metal roofing, ….etc,.

Yep. Nothing cheap. Folks who haven't built something in 2 years are shocked by prices. But they are what they are.

CrazyMRanch said:

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I've got a nice 25x35 pole barn with a dirt floor. I'd like a more permanent, and more clean, concrete floor. Any ideas on what that might cost, and how thick I should have the floor?

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Budget about $7/sf for 6", 4000psi concrete

As another poster stated above - get your existing subgrade compacted, and compacted well prior to doing anything with concrete. Bad subgrade means bad concrete no matter what. Depending on where you are, a vapor barrier may be needed or not - in my area of the state, it is needed. If it isn't, just prior to placing the concrete, take your hose and wet the ground and reinforcing steel so that the subgrade doesn't wick moisture out of the concrete right off the bat.

Use actual reinforcing steel, not remesh. #4 minimum, #5 preferred. 12" OC EW, have them use chairs or dobies to keep it centered in the slab.

4000psi concrete, it isn't much more expensive than 3kpsi, but it is much better. 6" thick would be my preference (and what I'm doing in my barn)

Make sure the contractor vibrates it in place, don't let them add a million gallons of water (more water makes it easier to spread and finish, but reduces compressive strength). Since it's indoors, controlling moisture loss won't be as critical, but at least have them apply a membrane cure over it after it is completed to control moisture loss - that helps prevent stress cracking from losing moisture too fast. Water cure is the best, but can be a pain.

Dont drive on it for 14 days! Some folks will tell you 3 days or 7 days....14 is the way to go. That pretty much ensures that the mud will reach its full compressive strength and design strength before you put an sort of dynamic load on it.

Depending on the type of barn, beams may not be needed. Pole barns dont require beams, so uniform thickness mat slabs are just fine.