it all depends on who you know, I think poured concrete is about $80 p/yard...

18 x 20 = 360 sq. ft.

360 x lets say .33 (4") = 118.8 cubic feet @ 4" thick

27 cubic feet in a yard so 118.8/27 = 4.40

4.40 x $80 = $352.00 + the labor for a couple people to build the footings. I'd say $500-750 depending on the plastic/wire mesh/reinforcing you do in and under the concrete.

I'd love to know how close I am though...