I'd have to see the specific breeding to judge but there's lots of reasons. First one that comes to mind on the $600 dog is the person didn't do much in the way of health testing or spend a lot of money to bring the pups into the world.
Some breeders ask different prices for different breedings - titles on the parents and what they think the quality of the pups is.
Venus of Crooked Creek 1/26/13
Otto von Hena-C 05/23/08
Morgan Donnermond 08/04/01 to 9/28/12 Long Live the Queen!
I have duct tape and I am not afraid to use it.