In the States, grocery stores regularly put things like milk and eggs on sale in order to attract customers. Here in Mexico, they don't seem to change the prices to attract customers. Instead they give you something else for free. For example, on weekends, Walmart tapes small bags of juice to all of the milk jugs. So you get this juice baggy thing for free when you buy the milk.
I guess this is good for people who want the juice, but it isn't a very motivating factor for us when we want the milk and don't particularly care for a small bag of yellow juice that we can't identify.
Another thing interesting about pricing, is that milk seems to cost the same at the corner store as it does at Walmart. There is also no incentive to buy in bulk. Milk is 18 pesos for 1/2 gallon and 36 for a full gallon. With the higher expense of packaging two 1/2 gallons over one full gallon, you would expect the full gallon to be at least a little cheaper.