You linked to a 7900GS. Which, while being g71 based like RSX, only has 20 pixel shaders and 7 vertex shaders active, is clocked lower and with slower memory.
So I think the $129 is probably pretty close to the real cost of a GPU with 256MB RAM mounted on a substrate similar to what mobile GPUs use.
Cheers
I'm sorry but the difference in pixel & vertex shaders "active" and clock differences between the two chips (it's basically the same processor!) will never make up the difference in cost for a whole board, a 256-bit memory interface and 256MB of GDDR3, all the different video outputs and the PCIe bus.
And if Sony are making the chip themselves, it's likely to be even cheaper for them to manufacture.