I really don't see the PS4 costing more than $400 at launch. The bluray tech has gotten much cheaper sense the PS3's crazy $600 launch and rumor has it, Sony is going to ditch the cell chip.
Both of these factor into a cheaper manufacturing cost and therefore a cheaper price for us. It is also rumored, that the PS4 will be much smaller than the PS3, at a size that is somewhere between the current PS3 and the Wii. Smaller size also reduces manufacturing cost.
Also, the economy (worldwide, not just the US) is crap. Not too many people can afford an expensive console right now. The Wii-U is rumored to be priced at $300, and the new Xbox is rumored to be around $400. If the PS4 launches at anything above $500, people may look the other way and go towards Sony's cheaper competitiors.