I talked to a friend who owns an anime store and is an avid gamer about this when I was thinking of going retail. I didn't ask the source of his information, but, based on his credibility, he's not one to bs if he doesn't know what he's talking about (and this is in line with what some others are saying)\:
New games (definitely new releases, not certain if ALL new games or JUST new releases) are 4% for your little local store, 8% for your Gamestop, Best Buy, Wal-Mart. They are only really used to get people in the door, which is why GameStop IS so hard on it's employees to offer pre-orders/EDGE cards - they're really not making money selling new games.
A "little local game shop", as a rule, always charges SLIGHTLY less (maybe $.50/$1.00) off MSRP and offers slightly more on your trades to compete with GameStop, b/c they're big and have name recognition, so you need a reason for people to come to your store.
For all the GameStop hate there is in the world, the fact that they push preorders and edge cards and charge ridiculously on used games is what keeps them in business and allows them to offer new stuff on midnight release, etc. etc. Even at the 8% they get, they're only getting about $5.00 PER new game - in order to stay in business, they need to make more money in other places.