Amazon has a deal for writers who decide to put their books up for sale in their e-books store. The concept is, that you get everything formated and ready to go on your own, and put it up for sale.
To do this, you choose a price point. If you sell between $2.99 to $9.99 you get 60%, but if you go over $9.99 you only get 30% of the sale, which means you need to sell it for over $20 to get an equal amount of money from the $9.99 sale.
To me, the over $20 sales would only go to technical books, that teach some programming language or other useful skills. That means, for fiction writers you would be deciding between a $3 or a $10 book to sell. The usual paperbacks, sell for $5.99+ if its been out for awhile, or $17.99+ if it is more recent. To compete, you'd have to go under that, so practically, you would be selling your books for $2.99-$4.99.
That means per sale, you're netting about $2 average per sale. For me, with both me and my wife working, I'd want to be bringing in, $1000 per month, which means selling 500 books a month.
With that in mind, an independently published book is considered "a hit" if it sells 2000 copies. Extrapolating from that, I would be a successful independent writer if I completed (wrote, revised, put together, and published) a book every 4 months, or 3 books a year (and that's if it was a success each time).