If you're paying a $3 royalty to the rightsholders per copy sold, then the marginal cost is at least that plus minor distribution and transactional costs. If you sell the e-book for $2.99, that's below marginal cost.
But that is not what the comics argue since they are attributing it to DOJ guidelines for the Licensing of Intellectual Property.
There is not a single reference to marginal cost or copyright infringement in that document. The guidelines focus around market power, and competitive prices.
Contrary to the comic, the guidelines do to define what an competitive price is, or public goods, or systems goods, or the effect of supply & demand on IP. The closest we get is a statement that market power exist if the consumer can not get the product beyond from a single source.
If someone edited the Wikipedia article about IP and used the same wording as the comic, it would get reverted with the comment ("Does not exist in source").
