If the 12K includes all excavation, compaction, forming, fill, compaction, steel reinforcing with dobies, concrete, concrete finishing, and finish grading after the fact it sounds reasonable.
You could do a relatively larger building for about the same price.
This is right-on. The problem here isn't the cost of the materials (obviously, from the other posts) - it's the labor and equipment. ... and the VERY expensive equipment operators!
With multiple pours, you have to pay for heavy equipment to be trucked out to the site several times, along with their high-dollar operators, etc. Even though your excavation may take 4 hours for the "small" garage, you will still need to cover the full day of work for those employees, the full day of rental/usage for the equipment, and the time/gas to truck all that stuff around from wherever its coming from.
^ All of this eclipses the cost of materials and/or the cost of labor do do form work, laying rebar, etc. If you have a contractor who decides to come in with gratuitous use of pumping trucks, etc. - that will really blow the budget. (Why should they have to be efficient when you are paying for it? BTW - they know there is little competition and you don't have a lot of choice.)
If you can sit down with your contractors and discuss their means and methods, you should be able to get the costs down. (I was able to get some of my house's concrete work cut in half by making schedule changes so we could more efficiently plan the concrete contractor's work, etc.) Whatever you can do to provide land access to chute concrete from the truck (instead of pumping), limit use of equipment, or trips for equipment and operators will have big impacts. If you have a neighbor who needs a garage done and you work together, (so the contractor can take care of both of you while they have equipment on the block) you could work out a huge discount.
