Marginal Cost or Benefit

The marginal cost and marginal benefit is the cost or benefit of the next unit of something. It is often easier or harder to get one more unit of something depending on how much of it is already being produced. Sometimes it takes a lot to produce the first unit (suppose, for example, that you are building aircraft), but much less to produce subsequent units (since you’ve already got all the tools and stuff you need to build airplanes). In other cases it gets more expensive to carry a process further. Thus, it may be affordable to get rid of a certain amount of pollution but very expensive to get rid of the last few drops of pollution.