It seems to be one of the most asked questions on all the writing forums these days, and it never gets an answer. Why? Because there isn't one. Not really. It all depends on the publisher, and the advance, and the sale price, and the marketing, and the genre, and the demand, and the list goes on. Too Many variables make what seems like a simple question, almost unanswerable.
I say almost, because now there is at least something to go on.
Lynn Viehl, a bestselling author, published the numbers on her book Twilight Fall online for all to see. I came across it today, and while interesting, the numbers themselves were not all that promising...
When Twilight Fall was published in July 2008, it hit the NYT bestsellers list in its first week, afterward selling nearly 81,500 copies in the first four months after its release. Ms Viehl earned royalties of nearly $40,000 in that period alone. Her publisher held back $13,500 as reserve against returns, leaving a royalty figure of $27,700—but as Ms Viehl was paid an advance of $50,000, she has a further $22,300 to make in royalties before this book earns out, after which time all royalties—minus her agent’s fees and tax liabilities—will be hers.
Just some food for thought.
(Thanks to Jane Smith of 'How Publishing Really Works' for the info and quote)