It does pay well. But it's one of the forms of writing about which I've heard the most horror stories: e.g., a celebrity decides after months of work that he or she doesn't want to reveal as much as the proposal promised and walks away from the book. So the celebrity and ghost are both out the next installment on the advance (which doesn't matter to the rich celebrity but does to the poor author).

You really need a strong contract up front that covers things like this.