A) 7800GT is built on 110nm, much more expensive to manafactur. 7900GT is built on 90nm, that's why it can be manafactured for $150 less than the 7800GT was.
B) Nvidia also has competition with ATI's models this time around, quite the opposite during the release of the 7800's, so I'm guessing they were planning a MSRP of $350 but had to cut it down to $300 to compete with ATI's models.
C) Nvidia also knows DX10 is around the corner, and needs to make these last string of DX9 cards appealing.
D) There is also a sufficient supply of these 7600's and 7900's, so there is no supply issue.
E) No reason in making "ancient technology" seem appealing when you have faster technology for the same price. If people find your product appealing, and awesome, rather than jacking up the price for a huge profit, they can lower it and expect more profit because they will sell more. It's basically Quanity>Quality.
There are a few more I could research or think hard about, but there's no real point. Someone else can add if they'd like.