Definitely "documentation beyond "how to use this" is going to cost you more" is a policy I do for side business outside my day job. That said I do try to spend a bit of extra time to make the API easy to understand, well named, and expose only those things they need. But I just consider those good coding and time well spent when I have to revisit it. What lies beneath is not always be so pretty, depending on what the customer is paying.