I agree with DavidRM, "The Ugly Truth is: There is no meaningful "average"."
My
free cookbook ebook is hugely popular because it is free, not because it is "one" of the better cookbook ebooks.
DavidRM has some great advice which every author should follow: "...use your own numbers as a gauge and see what you can do to improve those. Find the points of failure, the points where your potential customers/readers abandon you, and experiment with ways to remove those problems."
Smashwords, and other ebook retailers, grant readers a sample of an ebook which they may read fully or abandon after a few sentences. This method assures there is no way to determine "the points of failure" DavidRM mentions.
Shifting through the sales report spreadsheet from Smashwords provides some information on what sold and when.
From that information I look for reasons why there was a large grouping of downloads/sales during a particular period.
Maybe it was near Christmas, maybe it was around the time I was doing a lot marketing on some of my ebooks.
It could simply be that someone loved an ebook and told a bunch of people who told a bunch of people.