I don’t have the answer to that question. But I have found a hundred or so authors are interested in finding an answer. And that’s really the key: an answer, because surely there isn’t a single answer.
Check out this video of Tim O’Reilly provided by the fine folks at The Open Publishing Lab at the Rochester Institute of Technology. In it, Tim tells how he has monitized publishing things for free, plus some clues on how others might approach the opportunity. It’s only 4:00. Allocate the time. And mad props to Teel McClanahan III for bringing it to my attention.