The other thing missing is that a 1960's hardback is a much higher quality item than most modern hardbacks- sewn binding, nicer paper, better cover materials, etc. Hard covers today are cheaply made from inferior materials.
Though inflation's really bumpy across categories of products (largely due to microelectronics tending to drop in price over time, often while also increasing in at least some measures of quality, during the past half-century or so) it's clear to me that it's a lot higher than generally reckoned for many specific goods. Yeah you can get stuff that's "the same" price, or maybe "only" 2-3x higher(!) after nominal inflation adjustment, but if it's also made with worse materials and processes, and getting one as-good as the historical example actually costs 10x as much as the supposed inflation-adjusted price... well, that's worrisome.
(To be fair, though, pocket "pulp" paperbacks of the mid century were generally terribly made, certainly not any better than the now-on-its-way-out mass market paperback format of today; it's not that every type of good was better-made in the typical case, back then, just some)