It's an interesting discussion to start. A lot of the epubbers I've looked into go between $2.99 to $6.99 depending on the word count.
The interesting thing is, according to
The Inflation Calculator, dime store novels cost $2.20 today.
I'm referring, of course, to the 1920's style dime store novels, or penny dreadfuls, or whatever you want to call them
But, along the same token..my 1948 copy of "Border Kid" by Max Brand would cost $26.00 by today's inflation.