Concrete costs will vary according to whether you are setting the forms and reinforcement yourself or not, handling the pour and finishing yourself or not, and whether you are doing anything other than weekday 8-4 delivery of whole truckloads of basic concrete. Saturday delivery is extra, small/partial loads are extra, distance can be extra (especially if you are tied into using a further company to get, say saturday delivery not offered by your closest), higher strength grades of concrete is extra, additives for air entrainment, pourability/pumpability, fibers, color stains, and antifreeze for pouring in winter are extra.
Forms are pretty easy to figure out, as is reinforcement (I'm a big fan of rebar on chairs, and there have been a number of similar discussions here on TBN if you do a search). Basically, don't rely on just fibers in the mix, or on wire being "pulled up." Saw pictures of a friend's pour last week in western ma, two guys doing an excellent job screeding her new garage slab. Standing on the wire. Screeding, so obviously not jamming a rake in that to pull it up. No chairs or blocks in sight in the "before" photo. Concrete is in and set, so too late to do anything about it now but tick her off.
Pole barn vs. brackets on piers vs. frost depth footings and stemwall vs, floating slab will also have a big affect on price. Some of those choices will be limited by your building inspector and your soil type. If you don't already know your soil, find out now. I am on sand, with a gravel base below that, with just a hint of clay. There is a working gravel pit I can see from my house. One street away, they are on ledge with a thin clay topping, vastly different. If your jurisdiction allows, pole barn is usually cheapest. Since this aligns with the least expensive building method, there is that. If doing studwall construction (like for steel), then you are likely looking at either full footings or reinforced edge floating slab, depending on what your inspector (and builder, soil engineer, etc.) are comfortable with.
Finishing can be a dark art, and managing the pour for a building the size you are talking about can be a nightmare in a hurry. My boss has a great story of a form blowing out mid-pour. But there are guys out there that do this on their own, not associated directly with the hauling companies, so it pays to shop this part around. I've also heard of people just hiring day laborers with concrete experience. To some degree, it will depend on your aversion to risk---reducing risk costs money, but is often worth the price...if you have it to spend. All my pours have been small, so managing them myself, screeding and a simple broom finish has so far worked for me...but I may have a concrete patio pour soon which will be an order of magnitude larger. Because of both visibility and scale, I will likely hire that one out. Likely the forms, too, since the existing block patio that I put in has some issues with drainage, and I'd like a better eye than mine to help address them.
That brings up another issue: Site work. Going to do that yourself, or hire it? Ideally, your slab top should be about a foot above grade, with a sloping door apron and the dirt around the sides sloping away for at least 4', more is better. Managing the flow of rain and snowmelt off the size of structure you're talking about can be a very significant issue. Plan for it now while you can. Especially if it means digging into a slope, retaining walls, and drainage tile. Again, lots of similar articles here if you do a search.
