I evaluate the price of food based on the price per calorie. People often seem surprised when I tell them this, because normally high calories are seen as a bad thing. However, from a purely economical standpoint, it makes perfect sense. If you maintain a constant weight and exercise level, you are going to eat a certain number, C, of calories per day. The daily amount you spend on food is therefore Cp, where p is the price per calorie. Minimizing p usually minimizes the amount spent.

There is an exception: minimizing p is not the cheapest strategy is when eating certain foods causes your weight or metabolism to change, reducing C by a larger proportion. However, minimizing p is a very good first approximation to the minimal price strategy. Maybe if you get most of your calories from cabbage, your weight will reduce (then again maybe not--maybe you don't have the willpower to stick with a cabbage diet, or maybe you'd just eat a proportionately larger amount of cabbage to get the same calories you had before). But because cabbage has a very high price per calorie compared to, say, bread, it's still not going to be cheaper even if you cut your weight in half with the diet.

Admittedly, minimizing p is very often not the healthiest strategy. Chocolate has a very low price per calorie, but you wouldn't want to base your diet on chocolate because of its low nutritional content. You'd be better off paying more and basing your diet on whole wheat bread or rice. However, this isn't a price issue, it's a quality issue. p is only intended to evaluate the price of food, not its quality. Quality must be considered as a separate factor from price when deciding which food to buy.

Another factor to consider is that high-quality healthy food with a high price per calorie may save you money in the long run due to saved medical expenses. You might argue that this means the healthy food is actually cheaper in the long run. That may be true, but it's very hard to quantify, while price per calorie is simple. So again, I prefer to put long-term medical consequences in the "quality of food" column rather than the "price" column. It's similar to how buying a high-quality piece of equipment might save a business money in the long run, but still be high-cost to buy.