The PRS2 has a beefier power converter in it for the newer S Series and works for both new/old. The PRS1 is better for older S Series (e.g. S170 and S270). I read a review a while back from this very site that gives details on this:
http://www.siliconpopculture.com/review/117_0_2_0_M/
Here’s one opening at $0.99 with no reserve:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=8789118401
And another starting at $10:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=8786944009
I mean it when I say don’t pay more than $100 total for one. If you’re not in a rush, then you can play the field and keep a watch for a cheap one. Since Sony released the VGP-PRS2 model the VGP-PRS1 is now being phased out. You won’t even find it on the Sony Style site anymore, so these sellers are liquidating them for next to nothing.