Concrete costs vary by area and by the quantity needed for the job and by amount of labor needed to complete the job. A simple rectangular slab would cost less than a dramatic curving multi-leveled design. Also is there direct access for the truck to the patio or does it have to be carted by wheelbarrow or pumped? Flat unreinforced concrete in my area goes for approximately $3.50+ a square foot labor and material.
I would suggest that you pick the two or three contractors that you had the best "feeling" about, perhaps the most helpful answering your questions, or most responsive, or generally offered the most information about how he would complete the job. I would ask those two or three for references and call those references! Find out from other homeowners if the contractor remained as enthusiastic about the job as he was when he was trying to sell it! Ask to see other examples of similar projects. It's a lot of work on your part, but not as much as doing the concrete work yourself!