I sold more than 4,000 books (closer to about 15,000), but have basically made a relatively comparable amount of money from royalties from the book.
That said, if you're writing a book looking to profit from the sales of the book, you're doing it wrong. I easily pull in six figures for speaking fees that result from having written a book. I wasn't getting those offers before having written the book.
Maybe fiction is a bit different. But you can always find a way.