Writer's Fallout Shelter

The mother of a friend of mine has recently finished writing a textbook and my friend informed me exactly how much she was making off of said book. To be honest, I was blown away. While I understand that the money one could make off of a text book and the money one could make off of a regular novel are in two completely different ballparks, it did make me curious. How much DOES one make off of published books.

I know that the money shouldnt be what writing is about, but you have to admit, if you know you could mkae a lot of money off of something, you might be a little more motivated to complete it. Is there any one here who makes a living off of published books? How much is average to make, and how many successful books would some one have to publish to be able to hang up their gloves as far as working a normal job goes?