In order to calculate portion costs of recipes, you must first determine the costs of your ingredients. For many ingredients, this is relatively easy. You just look at your invoices or at price lists from your purveyors.
Many recipes, however, specify trimmed weight rather than the weight you actually pay for. For example, a stew might call for 2 pounds sliced onion. Let’s say you pay 24 cents a pound for onions, and to get 2 pounds sliced onions, you need 2¼ pounds untrimmed onions. In order to calculate the cost of the recipe correctly, you must figure out what you actually paid for the onions. In this case, the true cost is 54 cents (2¼ lb × $0.24 per lb), not 48 cents (2 lb × $0.24 per lb).