The best example of marginal cost is the money paid in order to purchase

A marginal cost is one that is actually an added cost to somethign you already pay for like a product or a service. In this case, the best example would be like paying a textbook for a science course because you already pay for the course as a service, so paying for the textbook would be an added cost. You don't need it if you can learn from somewhere else, but it sure helps to have it.