Another factor to consider are the options it has. Take a look under the hood there should be a white Porsche tag about the size of a business card and on it will list several 3 digit codes the go to this site
http://www.kindel.com/Porsche/options.asp it will tell you what the factory option codes are the you can go to kbb.com and see what blue book value is. That way you can get an idea what that car is worth and negotiate from there.