Completed auctions for expensive games:

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=8158143575

http://cgi.ebay.com/ws/eBayISAPI.dll...157862633&rd=1

http://cgi.ebay.com/ws/eBayISAPI.dll...156572839&rd=1

(Or if you have an ebay account just search for SNES under games, with completed auctions to see what games have sold for in the past, sort it by highest price first)