I'm getting 12000+ now for best selling vaudeville novels now, and my sim is a vaudeville master. For me, the flat rate isn't the critical piece of the puzzle though, it's royalties per page, so if my children's books are paying at a rate of $7.50 a page and I'm spending as much time writing them as I would writing masterpieces, then that 2000 page masterpiece would have to pay in the neighborhood of $15,000 to be as good a return on my time investment as the children's books provide.
I'm going to try a game with three writers plus a utility sim and give each of them a different specialty: children's books, vaudeville and masterpieces to see which strategy leads to overall better results.