Then there's the issue of fiction versus non-fiction.
If non-fiction, what credentials does the author have to support the non-fiction title and what platform (i.e. prior articles on the same subject matter, regional speaking engagements, etc.) does the author have?
What platform does the fiction author have (i.e. what sort of social networking or other related activities (e.g. reviewing)) have they done? I've seen not very good writers build a support base hanging out on reader loops, forums, etc. When they launch their work, they can be very successful for at least the first few books.
My established pen name is comprised of previously released titles...it only makes me about $30 a month on my 8 or so amazon dtp items. My unestablished pen name that has only been around since December, has only 3 titles and has not been co-marketed with my established pen name makes over $300 a month (mostly from one book). I don't spend very much time platform building for either name because it's proven time and again not to be a factor for MY work. Everyone's mileage varies.
Genre along with quality and some luck are the biggest factors, I feel, for a new author with fiction releases.
If you're an unknown author looking to make money from self publishing and self promotion you will need to do a bit more than just publish on Amazon's DTP.
You should look into getting on other platforms. Services like Smashwords can make this easier. You may also want to look at print on demand services so that you can at least offer a hardcopy of your book.
You'll want to setup a website, either a website for yourself (or pen name), or even better also another site for each book. You can also setup a Facebook page and Twitter account (and update them regularly).
I would say, on the DTP specifically, you could make anywhere from a couple dollars a month to a few hundred per title depending on its reception and success. Like I said though, the DTP should only be one of many revenue streams.
I submit the most important factor is an intriguing summary/definition for both fiction and nonfiction. If your summary is boring, you are not likely to sell many copies. You gotta hook 'em and not let go.
So many good writers miss the mark when it comes to writing their summary.
On my very first book I only made about $20 - $30 per month. Only once I started publishing more books and changed the book covers to something more enticing (I write erotic/paranormal romance), did I start selling more. With 4 short stories and 5 novels out there I currently make between $1200 and $1500 per month. I started publishing in April 2009, so as you can see, it takes a while to get to that level, ie. your books showing up in the right searches and moving up the sales rankings.
I also publish on Smashwords, but only started that a couple of months ago - I would definitely recommend it, since it will get your book on Apple, Barnes and Noble and Sony and a few other sites. Don't know yet, how much I'm selling through those retailers, since they haven't reported yet.
Hope this helps.
What others are saying is true: it depends on the genre, book cover, blurb (think how they advertise movies - you've got to hook the audience immediately), and pricing.