You're ignoring DC comic book pricing?
Going with Detective Comics, the June 1969 issue (#388) was the first to go with the regular 15¢ cover price. The August 1971 issue saw the price jump to 25¢, but that price was billed with the tag "bigger & better". (The "bigger & better" was that in addition to the new stories, DC also included reprint stories to try and help justify the price increase.)
With the July 1972 issue (#425), the cover price dropped to "Now Only 20¢", and DC stopped including extra reprint material.
Detective Comics stayed at 20¢ through issue #437 (cover-dated October-November 1973), then jumped to 100 pages for 50¢ with the following issue.