At just 6" thick, and just for the floor itself, at 85 bucks a yard, you are talking 28,000 bucks for the concrete alone.
Add a footer broad enough and deep enough, and maybe a thicker floor, enough re-bar and wire ( remember #4 re-bar is about 5 bucks a length now, mesh is right in line ) add any excavation and grading, and I'd say your high estimate is on the low side.
Add the labor and you are gonna have a broke contractor.
Jump on it and cross your fingers.
Do the calculations yourself with material costs in your area. And, remember, materials are going up, not down, for the near future.
Cheers,
George
BTW, I've had a bid of 5600 for my 1200 SF drive, 4" thick, no footer. Your job is 15 times larger, 10 times higher. Bargain!