That's not ridiculous at all, that's "normal" price for ebay, I expected a lot more.
Check this:
http://www.ebay.com/itm/MINT-3dfx-Voodoo-4-Voodoo4-4500-PCI-FOR-MAC-32MB-Videoca...Sure, it's not a very common card, and it's boxed, but 1300 USD... Now that's ridiculous.
How about this one?
http://www.ebay.com/itm/VOODOO-5500-BRAND-NEW-IN-ORIGINAL-STATIC-BAG-WITH-3DFX-S...V5 5500 AGP, nothing special - 500 USD.
And one last example:
http://www.ebay.com/itm/AGP-card-3dfx-210-0364-003-Dell-Voodoo-3-1-10-STB-VGA-00...121 USD for the most common 3dfx card, now that's an insult.
Unfortunately some people are very greedy and they think they will find a fool, most of them probably never know what 3dfx was, they only know that it's "vintage", "rare" and *expensive".
Those prices aren't really a problem, they won't sell the cards to anyone (except maybe a very rich collector that's just starting to collect and has a lot of money to spend and no cards in the collection yet).
The problem is that by asking those prices they will encourage other people to ask the same, it happened to me here, people asking huge prices on really common 3dfx cards because they see they "worth" a lot of money on ebay.