This feels apples to oranges. There has to be a "cost of inventory" consideration that applies to non digital goods far far stronger than to digital ones. To the point that expecting electronic book sales to give any insight to durable goods feels farcical.

Similarly, the marginal cost of a new release has to include all of the creative production of it to that point. Authoring and editing. Not just the marginal cost of replication of a completed work.

That is, the marginal cost of a new release is creating the new release's content! Which is very very different from the marginal cost of just making a copy of it months later.