Watch ebay auctions to get an idea of the going prices for many titles. This will at least give you a ballpark idea. Some titles are quite popular while others are not which is reflected in prices paid. Condition is also important.
Disregard the "buy it now" only listings because they're generally crazy (I'm not sure what the sellers are smoking).
Disregard the "buy it now" only listings because they're generally crazy (I'm not sure what the sellers are smoking).