The two systems offer very different business models, but it is possible to make some comparisons.
The first thing to remember is that commercial publishers pay the author up front, while a self-published author makes all their money on the back end.
The way it works in commercial publishing is that the publisher tries to predict how many copies the book will sell in all formats and calculate the royalties from those numbers. They use that figure to determine the "advance," or more properly, the "advance against royalties."
This money is paid to the author regardless of how many copies the book sells. If it sells more, the author gets the additional royalties on those additional copies, if they sell fewer copies than predicted, the publisher eats the loss.
So, the author gets paid for so many thousand copies whether they sell any or not. Advances vary, but they're usually at least 5-10,000 for a genre novel from a major house and can go much higher.
In contrast, the average ebook earned
$297 from sales in 2012.
It's no contest. The average commercially published author took home in excess of ten times what the average ebook author (whether commercially or self-published) generated in sales.
The catch is that it's difficult to get signed by a commercial publisher, and the process can take years if not decades.
In contrast, self-publishing is open to everyone; and that $297 average in the first year is a lot more than you can hope to get from submitting a manuscript that keeps getting rejected.
The royalty numbers are much better, too. As I said earlier, the 70% on a self-published $2.99 book is about three times what an author gets from a $7.99 paperback.
You get far more money per copy, but earn less per title. However, if you can keep a popular series running through self-publishing you can get a steady stream of income that can easily rival or exceed the average for commercial publication.
If you are willing and able to take the time to build a career and can catch the wave, you can leverage the larger royalties into real money.
The one thing to remember is that while it's much easier to make less money than a commercially published author through self-publishing, it's not easier to make a lot of money. It can be done, but the big numbers aren't on anyone's side.
If a self-publisher sells 1000 books for $2.99 they'll take home $2100 - but the average book sells fewer than 150 copies.