Kevin,
In order to really succeed as an editorial illustrator, you’ve got to work for BIG magazines because they’re the only ones with budgets large enough to support quality illustration. That also means that the competition is fierce.
However, if you can sell prints, cards, and posters on a Web site (or even in shops), you can double-dip on the income from a given piece of work. A good example is Joe Tomelleri, whom we use for all our fish illustrations. The guy is world-renown for his ability to render the exquisite details of native fishes, but he’s still selling paintings, prints, and cards on his site ( http://www.americanfishes.com/ ). He even puts out his own catalog. I have no idea how much he earns from it all, but it’s his full-time job.
Good luck,
Phil