I got my final drive off ebay, chromed, new (the drive wasn't used previously, it had never been mounted on a bike) for 500. So that's the MOST I'd expect to see them for. You can probably buy a used, good drive for 200-250 or maybe as high as 300-350 for a perfect condition one with no chrome.
Some guy JUST sold one on the OA board, I don't know how much he got, but he had the ad deleted when it was gone....and I saw one NOT sell on ebay... the final bid was like 250 and the reserve wasn't met. There don't seem to be any for sale today on ebay.