You are viewing this from the perspective of how much it costs to produce a unit, but you are missing how much a unit costs to keep around once it is produced.
In the case of print books, keeping them in stock is basically a liability: you already paid to print them, and now they are sitting in a warehouse and it costs to keep them there and move them around. So, once the book has peaked its popularity and going down, you want to get rid of the print copies as soon as you can and are willing to sell them under list price. In the case of ebooks, the cost of keeping them around is negligible, so you don't mind waiting until people pay the list price.
It is the same thing with movies: Discs (DVD or Blueray) are ~20 USD when launched, which is similar or probably a bit higher than digital editions. But after a few months, you can get the discs from a bin for less than 10 dollars but the digital edition still costs the original price.
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