Since when? That is the relevant point.<p>Book prices have gone through periods of low-inflation and of high-inflation over the decades. A paper-back book today will cost you in the mid-to-high twenties of dollars.<p>When I first started reading paper-back books in the mid 1950s, they cost 30 cents each. Ten years later, in the mid-1960s they had risen to about 45 cents - quite a large jump in percentage terms, but still miles away from today's prices.
There have been a lot of changes in book production in the past few decades. Typesetting and prepress is now almost completely paperless and filmless. Much work has been outsourced to India. Files go more or less directly to press. Makeready is more efficient. This means that the first copy off the press costs less then it used to. Many publishers use cheaper grades of paper and binding materials than in the past.