Since I don't what people are asking for on ebay is "fair" or accurate, I just wanted to hear your expert opinions 
Oh and how do prices differ between AGP and PCI?

Queso explained very good the situation, I only want to add one more thing: it depends a lot if the card is bulk/bare or if it has the original box and accessories. Bulk cards are cheap, VGA cables are also cheap and common, SLI cables are harder to find, but they aren't expensive. But full boxed cards can get very expensive.

Also the model of the card is important. Bulk Voodoo2 cards made by Diamond or Creative are quite easy to find and aren't very expensive. Other brands (Hercules or Gainward for example) are harder to find and they are usually more expensive. Some exotic brands are very hard to find and they are quite expensive.

Yea, the box makes a huge difference...I was bidding on the mint hercules voodoo 2 that had everything included, but gave up when it passed 150. It sold for 250 shipped! Let's not even think about the obsidian x-24, That thing sold once (a few months ago) for 600+. No thanks! haha

An unboxed one will indeed be the cheapest way to go. I think some people got a hold of "new old stock" somehow. They seem to never have been used because mine arrived minty fresh with no scratches on the pci connector.