It's an interesting question.
I found a link that may be helpful.
Choosing 1884 as the "canvassing year" for subscriptions
prior to the February, 1885 U.S. publication of Huck Finn,
the following figures came up (based upon the latest full
calendar year, 2006):
$ 2.75 = $58.30
$ 3.25 = $68.90
$ 4.25 = $90.09
Can this be true?
I'm not so sure there would be many takers today, if
these numbers are accurate! (They are based in part
upon CPI data.)
The historical economists among us with better information,
or a more reliable way of comparing, might help set the