http://calculator.net/concrete-calcu...tubequantity=1

Figure minimum 3 yards according to this calculator. based on 12'x14'x6"

i called on concrete about a month ago. (it is a commodity, so it changes)
it was $96 a yard delivered to my house. It could be cheaper for you since you probably are closer to a batch plant than I am. Of course this is just delivered cost. you would still have to form it up and do it yourself.

myself, I would just get 150 bags of mud from Home Depot and pour it myself, it is easy and saves money for ammo. It would take longer for me to go to town and get the concrete, than it would actually take to pour it.