Brenda Hiatt has a "show me the money" section on her site which is well-researched and accurate. For example:
Romance novels, for Avon/ HarperCollins earn out at:Average advance (first book): $13,500 Average advance (subsequent books): $24,000 Advance range: $3500 - $100,000 Standard royalty percentage: 8% Average earn-out: $26,000 Range: $7000 - $100,000
A small advance for a nonfiction book would be $20,000. Of course, your advance may be much more, depending on the projected sales and how well your agent negotiates.
There are many, many variables when you write a book. At the upper end of the pay scale, you've got JK Rowling, who's making billions from her Harry Potter series and the spin offs.
At the other end of the pay scale, you have authors who make a few thousand per book.
Publisher's Marketplace reports on book deals daily, in their Publishers Lunch.
So, how much can you make? The short answer is that it depends on the book. It's the long answer too, because as stated, there are many variables.
On my new membership site, we cover researching book deals and researching the market for YOUR book before you start writing. There's no point in writing a book for which there's no market. So if you're interested in writing a book and making money from it, join.