Oh, no, I think we need to add the cost of the device to the total. Amortized cost, based on a life of, what, three years? five years?
By my reckoning, a five-year life on a $300 machine is not unrealistic, resulting in a cost of $60 per year, or a cost per day of $0.16. Since I read an ebook in two or three days, a generous estimate would be $0.50 per book. Added to an average cost of $8.35 for an ebook, come up with maybe $8.85 per book.
Since I read approximately 150 books per year, my cost would be $1327 roughly. Say, $130 per month. If I bought hardbacks and paperbacks, my cost would approximately double to $260 per month. So, my readers make it possible for me to half the cost of my reading, have more convenience and portability, and avoid the dusts and moulds and chemicals that bother me when I read old books.
Sounds like a good deal to me.