The concept of cost is tricky. People talk as if products have costs, but that is shorthand easily leading to confusion. Choices, not products, have costs. The cost of choosing to make one more unit of a product is sometimes called marginal, or variable, cost. The cost (per unit) of choosing to produce at a fixed rate for a year is called average cost. The cost per unit of choosing to build a plant and produce at a fixed rate is called long-run average cost. The cost of filling a rush or special order has no particular name, but it clearly exists. There really is no such thing as the single
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