Derek Canyon has been providing monthly stats of his sales for the past months. He openly breaks down the numbers of each of his four titles and provides an average income.
It's an interesting read, as Derek is a "new" writer, self-published, having sourced his own editing and book cover designs and is projecting around $14k per year from his current titles, based on to-date data.
That sort of money is not shabby at all when you consider the likely advance for a traditionally published novel for a new writer. Let's say $5k, $10k or even $20k as an advance against royalties. This is a one-off payment, and by most accounts, novels generally do not sell enough copies to provide the author with royalties beyond this payment. Compare four titles at $10k per, or $40k total, and Derek will have made this amount in 3 years and still have all the rights and control of his books forever.
Intoxicating stuff. Of course, I doubt most new writers who are self-publishing are making anything like his sales of 30 - 40 per day, but it is definitely an achievable target.