Sony spent quite a bit on R&D for the Cell used in the PS3 and it came with Blu-Ray, which at the time was very expensive. This is what drove the cost up. There was probably some hubris mixed in there as well after the runaway success of the PS2.
PS4 is using what is essentially off the shelf parts from AMD. There's no reason for it to be much more than $500-550, if it's even that much. Also, current rumors are saying there'll be two variants, one for $429 and one for $529. The difference probably being HDD size.