Collectorz.com allows you to add a value for each game, but as stated, as the values fluctuate wildly this wouldn't really do it. It would need to be linked to a database of average sale prices to be any use. But would also be dependant on condition of box,cart,manual, etc, etc. would have to be a very clever bit of software linked to a huge online database, and not sure where the database would get its pricing feeds. Or how often.