Small price changes don't necessarily always impact purchases, as in the example of the diner the other poster gave. You also wouldn't pass up a gum stick if it was 57 cents instead of 56, and you wouldn't buy more if they were 10 cents. There was a term for this in my economics class but I don't remember, essentially elasticity of demand is not always a perfectly continuous function in reality. Somewhere like Wal Mart might have the price figured out perfectly by their team of researchers but probably not most non-chain restaurants.