Say that someone contacts you via e-mail or using a different method on the internet and offers you money in exchange for writing him a book. Exact method of payment would be discussed, etc. For example, you could demand to be paid by each chapter or instead get half of the money up front and the other half after completing the book. In this particular scenario, assume that the client wants a novel-length urban fantasy story. For the sake of the thread let's assume a 100k word count (Prisoner of Azkaban had around 107k words, so that's about it). The glient gives you the initial premise and some elements of worldbuilding as well as a small list of things which should not be included (like for example rape scenes). Would you agree to such an arrangement and if yes, how much would you charge?