how much money do writers earn?

I would imagine that this depends on several things, such as the book, the agent, and such. It's difficult to know exactly how much a writer may earn from their book.

it is not easy to calculate the potential earning of a writer. it depends on many factors. however, if one has built his reputation as a good writer and built a considerable number of following, then earnings will definitely be higher. stephen king, dan brown and jk rowling have built up their following that any release from them will certainly earn a bestseller in the bookstore. start with a blog and let the world know that there is a good writer out there.

I believe it depends on subject of book, agent, publishing company and of course how good of a writer you are. Publishing companies publish only certain genre and all publishing companies have their own prices.