Depends, some take DDR 1 and some take DDR 2, speeds differ from 400mhz to 800mhz, go to http://www.lavalys.com/ and download everest ultimate edition, it will tell you what RAM and speed it is, then we can tell you what price it is, or you can check yourself aye
Also, the physical characteristics are important, is it 140, 144 or 200 pin memory (Usually you can find this info in the manual or from the manufacturers website)!
For prices, I would do a search on my site (I list a few online stores prices, it will give you a good idea of how much they cost):