Since some folks are still looking at this, I'll note that a successful SF book from a major publisher may only sell 30-40k copies over its lifespan. I believe

Since some folks are still looking at this, I'll note that a successful SF book from a major publisher may only sell 30-40k copies over its lifespan. I believe it's Prime Palaver #6 where Eric Flint discusses some of this in relation to Baen's Free Library. With numbers like that, it's not hard to believe that a little-known author may only sell a few thousand copies, many of them to dedicated followers.