I happened upon these while perusing eBay. I'm not suggesting anyone buy either of these, of course, but I found them (and the extreme price difference) interesting.

Used tank for $102, and $162 shipping, total $264. The picture either shows the underside of the tank, or it's not a Vanagon tank. 

http://cgi.ebay.com/ebaymotors/1985-VW-VANAGON-FUEL-TANK-1983-1984-1985-/380252093987?cmd=ViewItem&pt=Motors_Car_Truck_Parts_Accessories&hash=item5888ca7e23

New tank for $46.98 and $49.99 shipping, total $96.97

http://cgi.ebay.com/ebaymotors/1990-1991-1992-VOLKSWAGEN-VANAGON-GAS-FUEL-TANK-NIB-NEW-/250510594205?cmd=ViewItem&pt=Motors_Car_Truck_Parts_Accessories&hash=item3a5398509d

The place selling the new ones has several listed for different prices, but the years that they list don't match up to when VW changed the fuel filler inlet size. 

Ed