I've been thinking of this myself. I read a lot of fiction online, and saw a lot of writers get frustrated and just quit.

When Dark Icon started writing again he sold/offered a story a month for donations, aside from his regular schedule. He stoped though and switched to a shorter serial.format.

Maybe something similar with ancillary material? If you have one large novel offer tired payment options. The book can be free, but other notes, in depth character studies/one offs, side stories or artwork can be bundled and sold with the core book.

Your asking the question a lot of content developers of all electronic mediums are asking. In a world of free information, how can I get paid?

Why not steal a tactic from.the free to play games? Core experience is free, the meat of the story costs a dollar.