I'm not an expert on this but my gut tells me that Microsoft/Sony/whoever, when specing the device will make a bulk contract purchase for their equipment. So although retail market for cpu/gpu/ram may be so much right now they'll be able to get better hardware at a cheaper rate than any consumer wishing to build at the cheapest price avaiable. I would hope at least that next-gen aren't building at 660ti grade GPU.
I'm also assuming that if you are building a device that has a multi-year lifespan you can factor in the ever decreasing cost of components. Maybe the price of the components on day-1 are $500, but day-730 $350.
MSFT has stated multiple times that their goal is over the lifespan of the hardware to break even on it. (So 1mil units at -200 and 10mil units with at +20)
How this works with devices such as Kinect though I'm not sure. It's about attachment rates (extra controllers, batteries, etc.) on the hardware side
