Well... to give you an idea, I can custom build a decent mid-range PC for around $500, this is just in cost of parts (this is getting some of the parts used, btw). Not a beast, but not bad either.
Now, in order to turn a profit, a console has to be priced higher than the bill of materials. That means the BOM for the PS4 is some amount lower than $500. It could be they made a deal with various manufacturers to buy parts in bulk at a lower per-unit price, so perhaps it would be approximately equivalent to my custom PC in terms of power. Either that, or slightly less powerful.
However, consider that the PS4 is going to have a long console lifecycle. If the last generation is any indication, it will be sold as-is for at least 5 years, if not 7 or 8. During that span, PC graphics will still be advancing at a rapid rate, and old cards will fall in price. That means I can continually upgrade my Steambox over time with better cards as they become cheaper. That, I think, is the main power of the Steambox.
I believe the PS4 is sold at a loss, not as much as the PS3 was but still a loss.