This thread is superuseful and interesting, thanks for sharing your thoughts and experience guys!
I don't have anything really valuable to add to the topic yet
Last weekend I presented my Ribs plugin on an exhibition in Moscow and had a number of conversations with other developers and sound designers. It seems like to everybody it looks as foggy as Boz wrote. Just trial and error.
Surprisingly, one experienced guy who wrote a lot of useful free stuff said that donations that he receives are enough to make a living. I guess one has to be in business for a really long time to reach this state.