Phantom
The cost of concrete this morning is $113.00 CY. Add maybe 5% for standard retail (cash and carry pricing) this is redi-mix delivered to your home. The average charge for a concrete contractor in my area is $236.00 and includes the cost of concrete. This price would be for a full finished slab 4" thickness, the rat slab uses less material and is only rough finished, not floated.
To determine how much:
http://www.carrollconcrete.com/Page/Calculators
Determine your total volume and do the math. The grading and poly could be handled by your concrete contractor, some will gladly do it (within reason, they are typically not site work guys) others will refuse.