I've seen one but I don't remember what the bike shop wanted for it.

After a quick google:

You can buy one brand new for $800
and a off brand equivalent for $400

I'm a good salesman, so I would ask $400, because SS/fixies do not have a great resale. They are pretty much a fun item to have

I highly recommended to search around Bikes Direct for something better for you. But if you cna get it for around $500 then enjoy, any more then I wouldn't pay up. But do not leave on a bad note, give him your phone# and tell him to call if he'd like to sell for your price