as Kyle posted above, profit on new games really isn't much. The big chains will make a little more as they will get a discount for bulk orders.
Games dropping in the slaes etc will be because of a distributor discount. This can take the form of the distributor just adding a credit note for £XXX on to the companies account & the retailer gets to chose which titles to apply this to. more commonly though the distributor will pass on discounts on specific titles.
Console bundles are usually worked out with distributors offering discounts on certain games, which is why you will see places offering bundles with specific games that you cannot change.
The freebies that some places give away with games on launch usually aren't free either. The golden hammerburst codes being given away by various retail chains for GOW2 release actually cost the retailer £1.5 each. Doesn't sound like much, but as they don't pass this cost on to the customer that nearly halves the profit made on a game. The retailers have these `freebies` & rely on selling more copies of the games because of the extra.
Supermarkets sell games at a loss on release most of the time. This is get people in to the store, their theory being that whilst you are there you will pick up some groceries & they make their money that way.
Pre-owned is where the money is made that keeps most games stores open.