Batman Family "Dollar comic" back covers
IJ account created, check. Joined S_D 2.0, check. Userpic, can't find the damn thing, heck with it I'll get one later. Here we go...
The relentless stagflation of the 70's was not kind to comics. 50 cents for a 100 page super-spectacular became 60 cents, then became 60 cents for a 64 page giant, and finally 60 cents for a 48 page "giant." (back then, comic companies were actually reluctant to raise cover prices, you see). Then came the "Dollar Comic," at 80 pages for a dollar, with all-new content. But 80 pages was just too much paper to be affordable, so after only a few months of 80 page dollar comics, the page count was cut back to 64 pages.
But again, this was an era when comic publishers felt a responsibility to deliver value for your dollar, so DC eliminated all advertising from the 64 page dollar comics. 64 pages with no ads was nearly the same number of story pages as 80 pages with ads, so they could tell readers that nothing had changed except a lower printing bill for DC. And it gave the artists the opportunity to create pinups for the back cover.
Batman Family was merged with Detective Comics around this time: here are the back covers to Detective 481-485.
(This was for the 40th anniversary issue of Detective)
The no-ads Dollar Comic didn't last long, of course: as stagflation continued, the ads came back, then the page count was reduced again to 48 pages (again with no ads), but this time the back cover was really a house ad in disguise (the "front page" of The Daily Planet, showcasing various DC titles being published that month). Dollar comics stayed ad-free with 48 pages for something like a year before the ads came back again, and shortly thereafter the "Dollar Comic" line was cancelled for good.
If I have the time, and if there's interest, I may post scans from the back covers of my Superman Family dollar comics later this month.