This is something that I think confuses everyone, are writers rich or not? Is it a career worth pursuing or should you stick with being a window-cleaner, like your mother wanted?
So, I've managed to nick the TOP SECRET publishing document used by agents and publishers to work out how much they should offer you as an advance on your next book. It's only a rough version (all I was able to photocopy from the office before the security guards chased me out) but should give you a ball-park figure of what you can expect to earn.
1. Does your book involve vampires? Lose $10,000.
2. Does your book involve zombies? Add $10,000.
3. What about angels (but you'll have to be quick as their value is dropping daily)? Add $12,000 this week, losing $1,000 per week hereafter.
4. Knights Templar? Lose $50,000 and put that pen down before you destroy the publishing industry!
5. Have you or any of your characters ever been naked on telly? Add $100,000.
6. Does your book contain a character who's first name is Harry? Add £500,000.
7. Is his surname Potter? Add another $5,000,000.
8. Is YOUR surname Meyer? Add $1,000,000.
9. Have you ever been arrested or shot at? Add $20,000 (a certain notoriety goes a long way but PLEASE OH PLEASE don't now head over to Afghanistan/Iraq/South Central based on this document!).
10. Finally, have you had any personal experience with space aliens (aliens are HOT HOT HOT!)? Add $100,000.
So, based on the above table, the best thing to write is a blockbusting paranormal sci-fi adventure featuring a space alien called Harry Potter, fighting a world of zombies with an angel girlfriend which you'll write while in prison after having be arrested for running naked through Prime Minister's question time.