I only buy groceries from Wal-Mart when I'm at college. On a $100 lot at Publix or Food Lion etc... I get for $75 at Wal-Mart. They continuously beat the surrounding grocer's prices with "Save on Their Sale Price!" and a lot of the grocer's are going out of business because of them beating their prices to a pulp. For instance, a pack of oreo's at Publix is $4, however at WM it is $2.50! Sounds like nothing, but for the avid oreo-eater its priceless.
I agree with the above poster about some prices being cheap while others are "normal". I watched a documentary on how they try to set you up with a good bargain on the end or in the middle of the aisle. For instance, a $25 dollar microwave will be sitting in the aisle and you think, "hey thats a great buy! I bet the rest of the microwaves are cheap too, so I can get a great microwave for cheaper". It happens a lot, and they make money like that.