Obviously, the answer to the question varies by the success of the author. But, I thought I'd relate my own experiences as a less than mid-level author with just a few novels published.
I started my self-publishing efforts back in September 2010. Back then, I planned to commit to 5 years of writing before deciding whether or not it could become my actual career. By late 2015, I hope to have 10 novels in print. The sales of those ten novels should give me a good idea if I'm successful enough to quit my day job and become a full-time novelist.
It's still a long time to 2015, but how am I doing now? Am I making good money? Could I survive on just writing novels? Not yet.
In about 20 months, I've published 3 full-length novels, 2 short story anthologies, 2 DIY books, and 3 riddle books. I've sold a total of around 15,200 units , earned gross revenue of about $15,200, with net expenses of about $7,000, resulting in about $8,000 profit (or about $400 a month).
Definitely not enough to earn a living, but it is enough to take a nice vacation each year.
But another data point you should know is how many hours I work on my novels. I do not have exact numbers for all my books, but I did keep track of my hours at the keyboard for a couple of them and I spend about 200 hours actually writing/editing per novel. This does not include thinking about the plot or outlining. I bet that adds at least another 50 hours.
So I'm going to assume 250 hours to finish each novel. How much does that earn me per hour?
For my best-selling novel, Dead Dwarves Don't Dance, I've earned a profit of about $4,300. So I'm getting paid about $17 per hour.
But that number does not count how much time I spend on marketing (blogs, facebook, twitter, forums, advertising, etc.). I have no idea how much time I spend on that, and it is spread out across all my books. So, let's say that $17/hour is the maximum I'm earning.
Dead Dwarves Don't Dance is about 74,000 words long, so it's earned me about 5.8 cents a word. Certainly not a king's ransom, but within the range of what you can get for writing for magazines and so on.
Unfortunately, not all my books sell as well as Dead Dwarves Don't Dance.
The story is not as good for my other two novels (The Elemental Odyssey and Where Magic Reigns). These are young adult adventure novels full of magical aliens and fun action. But while I think this series has lots of potential and the books are getting good reviews, they have not yet found a large audience. My profit on those two books is actually a loss of $4000. Fortunately, ebooks are forever and I have no more large expenses for these two books. So that loss will decline over the years, and hopefully become profitable at some point. All it takes is one person of influence to find it and mention it to some friends, and the series might skyrocket. (That's what I'm hoping for, anyway.;)
On the other hand, my DIY book (Format Your eBook for Kindle in One Hour) has sold very well and had very little in the way of expenses (about $7,400 profit). So, let's average all my books together and see what my totals look like.
Out of all my books, I've earned about $8,000 in profit.
With three novels and seven other shorter books, I figure I've spent at least 1,000 hours working on them, for a maximum of $8/hour.
Not enough to live on, alas. But, the good news is there's room for improvement. ;)
Now, what can YOU expect to earn? Absolutely no way to tell. You could might earn more than me or you might earn less. It depends on how good your books are. But, even more, it depends on how much luck you have. Based on the success and failure of other books I've read, luck seems to be the overriding factor.
If you have a terrible cover or blurb, or you write like a 2nd-grader, or your story is pathetic, you probably won't succeed. But if all of that is polished to an acceptable quality, your success hinges pretty much on luck. It all depends on getting discovered, and in the overcrowded ebook market, getting discovered is just luck or thousands of dollars in advertising. Not many of us have $$$$$ to spend on ads, so we rely on luck.
In almost all careers, you have to commit time and earn your way to the upper echelons. Sweat out the low pay early years hoping to improve your results in the long run. Fortunately, I have another career that can support me while I write novels.
My advice to anyone else is, don't quit your day job. Write diligently and in several years you might start earning a wage large enough to support you.
Or, you might be lucky and have a blockbuster on your first try. If that's the case, congratulations! It's kind of like winning the lottery but at least you earned it by writing a book!
If you're an author, do you have any guesses as to how much you're making per hour/word? Put it in the comments.