What is the marginal cost of a digital good? The cost to download it one more time? It seems like the marginal cost to produce one more copy of an existing digital good is close to zero.

Books and music and other similar works certainly have a large fixed cost to produce: the opportunity cost of the creator's time; the technology or equipment involved. The producer might choose to market the work, which incurs further cost (but it's not marginal). However, once the work is created, can it really be said to have a marginal cost beyond the cost of digital distribution?