The builder recommended concrete contractor that I went with quoted $5.25/sq ft (1,080 sq ft - 30x36) for a 4", 4,000psi mix with a rebar reinforced perimeter beam and 3/8" rebar on 12" centers. I paid him an additional $1/sq ft for a 5,000psi mix, with rebar forms in cross beams (both length-wise and width-wise beams in addition to the perimeter beam) and two 3'x3' pads for a future 2-post lift. They did a quality, professional job and cleaned up after themselves. I was happy with their work.
I do not know if the numbers above are his standard or discounted, as he does a lot of work for Morton Buildings, who should be putting up my building in a couple of weeks...
Juan is based out of the Waco area, but he and his crew do jobs all over this area. I'm just North of Austin.
Juan Hernandez (25four) 7two3 - 8five22