This whole article is begging the question. Books aren't expensive because they could cost more. If they thought people would pay more, books would cost more. There could be many reasons books didn't keep up with inflation - their production might be cheaper due to efficiency or cheaper raw materials, maybe they were already too expensive, or maybe inflation is skewed towards certain areas - imagine using the same argument to say RAM isn't too expensive actually because of the price of it in 1980.

It's interesting that he didn't breakdown the cost per book to the publishers. I think before ebooks came out he probably would have done, but ebooks have made it clear that books are priced at essentially the price they think they can get away with.