Any time someone is paid based on the number of units sold (e.g. royalties), sure it is. If I sell one more ebook, I need to pay out more royalties, and therefore my total cost has gone up.
I realize I'm handwaving a bit, but my basic point is that selling more ebooks is not free for the publisher. There's definitely a cost here. And it's somewhat tangential anyway, because the marginal cost that's important here is Amazon's. In the traditional model they buy ebooks from the publisher at a fixed price, just like with physical books, and then mark them up and resell them. So the marginal cost here is the amount they pay the publisher per copy.
I realize I'm handwaving a bit, but my basic point is that selling more ebooks is not free for the publisher. There's definitely a cost here. And it's somewhat tangential anyway, because the marginal cost that's important here is Amazon's. In the traditional model they buy ebooks from the publisher at a fixed price, just like with physical books, and then mark them up and resell them. So the marginal cost here is the amount they pay the publisher per copy.