Well, the question of 'What Should Authors Expect to Earn?' is the title of an article by Brooke Warner, which was published on The Huffington Post website and addresses this very question. In her article, Warner writes:
I see two types of independent authors: The first batch are those who have a legacy project they want to publish beautifully and be proud of. They want their book out in the world and for it to have a fighting chance at getting reviewed, being carried in bookstores, and selling a decent number of copies (maybe 1000 or more). These authors have a very good chance of achieving their goals. The other subset are those who aspire to be career writers. These authors expect to earn out their expenses and to turn a profit on their investment. They want the other things too--to be in bookstores, to be reviewed, etc.--but their expectations are generally higher, and wrapped up in money, because earning out and selling a predetermined number of books is tied together with their perception of success. Thus, more is at stake, and they expect to succeed.
If you're in this latter category, there's good news/bad news. The good news is that it's possible to earn out your expenses and turn a profit on your investment. The bad news is that there's a very slim chance that this will happen with your first book. You need to keep creating content and stacking up more product (in the form of new books) to generate five-digit sales numbers. Very few independent authors reach 10,000 sales, and when they do, they're typically scooped up by traditional publishers. Rusch makes the incredibly good and valid point that the most successful type of authors are those who are publishing every which way they can. Sometimes called "hybrid authors," these are writers who publish traditionally, who self-publish, and who publish in between--perhaps through other independent models, or perhaps publishing some digital-only books.I found Warmer's article fascinating. I hope you do as well. Click on the above link to read the full story.