Well, let's see... A £40 game typically has a day one trade in price of about 36 pence... 34 games, probably not brand new, so half it... so about £6. A good option, if you have the time, is to start a price war between multiple stores and see how high you can raise the amount. Two competing stores, each with a price match guarantee? Easy money.

Or you can use the maximum offered as a base, and then sell them on ebay or some such for more than that.

If you just wanna get rid of them quick, an average of £5 per game is usually fairly accurate, and I assume American stores use the same tactics. If they don't offer you more than $50, just rob the store.