I still have paperbacks with prices printed on the covers of a dollar or less... but I'm getting pretty old. Books back then were more the size of Nobots than today's books. Of course, back then a gallon of gasoline was under fifty cents and a six pack of beer was about a dollar.
These days, judging by what I've seen, new paperbacks range from $6 to $10, so $7 doesn't seem too out of line.
The library here has a book sale every year, and I picked up a huge writer's guide for two bucks. Its copyright date was 1978 and it was completely obsolete. Along with talking about typewriters and carbon paper and estimating the number of words, it stated that publishers would rather publish two 40,000 word books than one 80,000 word book. When Twain was asked how many words should be in a book he said "as many as it takes to tell the story, and no more."
How times have changed!