Book sales are a funny thing, because we get rankings, but not unit or dollar sales (those numbers are apparently proprietary to Bookscan).
But here's a formula I found that supposedly works as a rule of thumb:
100,000 / Amazon Rank = Number of copies sold per day.
(Note that this is overall rank, not GN or Marvel only).
Which means that currently, the great critical darling TNC is selling a grand total of about 7 copies of the first trade a day in soft or hardcover. Or ~200 a month.
Doesn't seem so great now, does it? He's doing a LITTLE better on the Kindle. There the first trade is selling... 3,000 a month.
Whee.