As I stated in other threads regarding this issue, only select laptops have the option of upgrading the video card. They generally costs $1800 - $2000 to begin with (at least) and not all of those will have that capability.
Additionally, laptop video cards are relatively rare and expensive. A few years ago they were actually a bit easier to find because I was looking into building my own laptop from scratch; laptop chasis, motherboard, video card cooling system... "the works". It was a little more expensive than buying a "moderate - high end" laptop, but it would have been to my own specifications.
I would not be surprised if a Mobility HD 5650 was selling for at least $200. As a comparison, the desktop HD 5670 can be purchased for approximately $65 - $75 after rebate.