Thw question probably is how much an Nvidia GTX 280M costs. But mobile graphics are not the same as desktop graphics. Currently only clevo makes laptops (barebone) that support the gtx 280m. If you are thinking of just upgradiing your laptop that is not possible
slo is correct, You can't just upgrade your laptop video card at your own free will.
Typically laptops don't have room for or may not be designed for a mobile video card. It's not like desktops where you can just buy a card a slap it in just about any machine with the correct port.