I think it's pretty unprecedented for an entertainment figure, much less a mere writer, to be among even the top ten richest people in the world. Stephen King, a writer who has had his work adapted into(usually quite lucrative) Hollywood films and TV shows close to a hundred times, isn't, as far as I know, ever included on lists of the World's Richest.
I think you'd need Lovecraft to become some sort of Walt Disney-type figure, with some of his characters taking on a highly iconic status, and being reproduced around the world in books, cartoons, movies, t-shirts, diapers etc, for this to work. And, even then, I don't think I've ever heard Uncle Walt listed among the world's richest people, unless the list is confined to entertainers.