Really depends on your system and what kind of RAM you need. Do you need SDRAM or DDR or whatever, and if its DDR 2100, 2700, 3500 ect... Go check out some of the memory makers websites like
Kingston you should be able to find out what kind of memory you need and a good idea of how much it would cost.
There are a ton of other companies out there besides Kingston it was just the only url I could think of off the top of my head. You could always do a Google search find out some more info or go check out pricewatch or something like that.