The oil crisis and the attendant inflation started around 1974.
Paperback prices had already doubled/trebled from $0.35/0.50 to $1.25/1.75 by the early 70's.
I have a bunch of used paperbacks dating all the way back to the 50's.
I could go digging through them to document pricing up to the 90's, when I started transitioning to ebooks. Once paperback prices hit $6 I cut back drastically on new purchases.
(Plus, the TBR closet was overflowing.)
Or, I can go see what Google/Bing cough up to back my recollections.
Edit: fbone already did the honors in 2010.
https://www.mobileread.com/forums/sho...d.php?t=102755