they change prices, but not due to a change in costs. they change prices for reasons other than costs.
at my local supermarket, split chicken breasts are 99 cents a pound, this week, last week, over 2 bucks, next week, over 2 bucks. prices set by supermarkets are not based on their costs. the supermarket is not getting split chicken breasts for half the normal price this week. they just aren't.