It's not a matter of cost. I have friends in the industry (tv writers and film writers), and there are SO many factors that go into the script. 1) Writing by committee, where the writer(s) go in and the producers each have different demands because none of them is envisioning the same movie; 2) Directors have more control than the writers, where no matter what is in the script, the director has a vision and wants everything to change to meet that, and 3) Multiple writers on the same script, where the first writer probably had a coherent idea but the producers, or directors, or sometimes even the stars (if they have that kind of pull written into their contract) keep wanting something different and so the script goes through sometimes five, six or more scriptwriters - each trying to please someone else and the script gets further and further way from what the original writer wrote (and which was purchased for production).