Costs incurred in producing one additional unit of a good. Marginal costs are calculated by dividing the change in total costs by the change in quantity.

In predatory pricing cases, courts sometimes use marginal costs to determine whether a defendant has priced below cost, an element of a predatory pricing claim. Marginal costs play a similar role in primary line competitive injury cases under the Robinson Patman Act.

When data on marginal costs is not readily available, some courts use average variable costs instead.