Fair price for 1989 Weingut Adolf Lotzbeyer

[jimpeterson]

Again, I must tap the knowledge of wine experts out there. When invited over to dinner, I got a chance to try an excellent desert wine called "Weingut Adolf Lotzbeyer" (1989 vintage) and may have a chance to acquire some. But I have no idea what a fair price is per bottle. Can someone help me out? Are there some websites that keep such info on older wines?

Thanks,
Jim