Around here it runs around $2 a sq. ft but with your footings and foundation, that isn't too bad. Concrete itself runs about $50 a yard. If you are parking any heavy trucks, I would think about thicker concrete but go with what is recommended by your concrete crew.
Also talk to your carpenter to see how long it will take him and if you help out, etc. You can still add more time for installing doors and siding. I'm not sure what all you are having him do. Go ahead and buy some 6 mil plastic for a moisture barrier under the garage. It may help later.