This is a tough question. It's really hard to say how much they pay per car. Usually cars are bought in bulk directly from the manufacture before the car is even made. The cars are order aprox 1 year in advance. Then the cars are sent out to the area's.
The big boys dont really buy to many cars, best term to use would be lease, they use the car for a certain amount of time or up to the mileage cap, which ever comes first. Once the car hits one of these it is not consider a TB (turnback) that means its time for the car to be turned back into the dealer. They go to a GM TB yard and that yard sends them to dealers.
Opps forgot to add something. To turn the car back the car must meet certain criteria. IE not over the miles, under certain about of damage/repairs that have been done. If the car is not fit for TB then its is considered a Wholesale and is then sold.