Don't forget that on that video card things can get that complex. IE running traces to 12 memory chips. When you're dealing with a system, you also have to run traces to the CPU. Then you add in a sata controller for the disc drive, and hdd. I/O controllers, USB controllers etc etc. Suddenly your motherboard complexity shoots right up and it becomes really expensive. That all adds to the cost. It may only add say 12 bucks to the cost of chips to add some more, but suddenly its costing you X bucks more in producing your motherboard because of the added complexity. That has to be added into the equation as well.
Plus to Nirolak, they may be aiming for a 400 dollar system losing 50 dollars at launch, but if you're suddenly spending 100 dollars on every machine in ram, that doesn't leave much room for the CPU, GPU, Disk Drive, Motherboard, Hard disk drive, Updated Kinect, etc etc.
A good console is a balanced console, and blowing all your costs on memory is going to lead to a very unbalanced machine.
*Edit*
I should have kept reading I see people replied to this stuff already, never mind me then.