I had my floor done in fall of '08. Size 30x48 (1440 sq ft). Mason charged me $1300 labor.
The $1300 price included: he helped a little bit with site prep (compacting, spreading gravel). He installed 6"x8" concrete block around perimeter on top of floor. He brought over the 20' rebar needed for the footings.
He did a great job. (35 years experience) I get many favorable comments on it. Looks good, and no cracks yet. He is a one-man operation, no crew to pay, and I was available to help him.
If you're going to have electrical in your barn make sure you check to see if your ground wire has to be hooked to a 20' piece of rebar in footing.