My personal experience knowing a lot of authors is that even a unknown author manages to sell at least a hundred or a few hundred books.
Unless you're trying to make a living out of authoring books, I would say that selling more than a thousand books is already extraordinary success.
This is accurate in my experience. My two self-published history books have sold about 400 and 150 copies respectively (combined print an ebook), with no marketing other than my blog, which occasionally makes it to the front page of HN.
Note that this is purely a 'hobby' project at this scale. Both books are still in the red counting just the expenses paid for cover and interior design, illustrations, and editing (i.e. not counting the many hours of my own time). To make a self-sustaining amount of money self-publishing I would have to sell a lot more copies and make books that can be written with much less effort (e.g. genre fiction).
A book might be 20 bucks, amazon or bookshop will take a cut, publishing house will take a cut, and there are costs to be borne, so even with a 1000 books you are not breaking bank, are you?
And I wonder if the textbook monopoly mostly favors the authors or companies?
In France, after the editor, the book maker, the distributor, points of sale and the state (VAT) take their part, the author gets only 11% of every book sold.
