Average Salary: $44,850
Expected Lifetime Earnings: $1,872,000
Illustrators range from highly successful to borderline destitute, so it's hard to nail down an exact average income. A deeper look into the numbers tells us there are around 12,000 people working as artists in some capacity, about 4,000 of which are independent illustrators. The average salary for all those different concentrations of artists is around $45,000 (source).
Now, if that includes all artists working for a living, you'll probably start out considerably lower on the scale before you have a large portfolio. The bottom line is you'll likely have to work part-time, and maybe even supplement that gig with a secondary job, if you want to make this illustrating thing work.
When first starting out, you'll be lucky if you can crack the $20,000 per year mark (source). Unless you can get hired by a company early on, you're more likely going to be building a portfolio through freelance work for employers who are willing to take a chance on an untested new illustrator with no prior experience.
It'll depend a lot on whether or not they like what they see, but if you've never done any work for anyone else, they may wonder why that is.
If you fast-forward a few years, you have the chance to join the top illustrators who make as much as $80,000 per year. That'd be nice, but it's still an unlikely mark to hit. Even less likely is that uber-rich level where you'll find people like Jack Kirby and Scott "I created Dilbert" Adams.
Whether working for a company or giving it a shot as an independent artist, you're really rolling the dice. You could hit the jackpot with a successful series of books or you could find yourself on food stamps.
Whatever you do, don't get your heart set on doing just one thing. Branch out and shop your talents around—eventually people will start buying.