The cost to produce the next increment of a product or service. When used in water and energy resource planning, it is typically expressed as the incremental cost in $/unit to produce and/or deliver the next (marginal) resource, whether a gallon of water supply, a kilowatt or kilowatt-hour of electricity, or a therm of natural gas. In regulatory policy, marginal costs are often used to evaluate the relative costs vs. benefits of various resource, infrastructure or operational choices.