It won't be too much because it uses the 1st generation core i5 CPU and most likely has the older Intel GMA 4500m graphics core which is rather weak compared to the current Intel HD 3000 graphics core found in current 2nd gen. Sandy Bridge i3/i5/i7 CPUs.
There are no Toshiba Satellite L640 currently being auctioned or available for a direct purchase. Back when it was new it probably sold for as low as around $600. I've seen a 2nd gen i5 laptop with the Intel HD 3000 graphics core selling for $500 - $525 when on sale. At best $300 is probably what you can get for it.