Compare prices of items on the shelves. Those which a placed at eye height are usually not the cheapest. Manufacturers pay the stores a premium to get their products placed at eye level. Look above and below to find other brands of the same product. The actual cost of many products is shocking when expressed this way.
Many premium brands use non-standard sized containers to make direct comparisons difficult.
Take a small calculator to help you to work out if one product is actually cheaper than another. When possible, buy in bulk.