Statistics for illustration income are hard to come by as many sources lump all artists together. The Bureau of Labor Statistics maintains their occupational outlook handbook online with the Arts and Design category here. "Through 2014, employment opportunities are expected to grow as fast as average", according to the Bureau of Labor Statistics , "though competition in all art and design fields is generally quite keen. The outlook is most favorable for multimedia artists as the demand for special effects in movies and television, animation, and video games increases."
Unfortunately, illustration fees in many fields have become stagnant over the past 20 years and in some cases are actually less than what they were 20 years ago. Every illustrator has times when there is no work coming in and some illustrators will occasionally have so much work at times that they have to turn it down. Book publishing ventures rarely produce royalties these days so the advance payment you receive as an illustrator may be the only payment you get. Beware of work-for-hire contracts as you will give up all your rights to the image.
Pricing is a complex art in its own right. There are many variables in determining pricing for illustration work such as complexity of the job, the amount of time available, and most importantly which rights are being licensed. You need a thorough understanding of copyrights, and your rights as a creator before you can begin to negotiate pricing. You also need to understand the various types of licensing and how that can affect your future earnings.
The best print resource for pricing illustration is The Graphic Artists Guild Handbook of Pricing and Ethical Guidelines.
While doing pro-bono work for a worthy cause is a noble gesture, it is foolish to do work for free or reduced rates for "exposure", "publishing experience", "a share of the profits" or "a favor for a friend". You need to value your own time, energy and creativity.
Mark Monlux has some great tutorials on pricing, negotiations, a sample contract, invoice tracker, and a project tracker .
Phyllis Cahill has some guidelines for pricing in the Children's Illustration market
One factor that may be in your favor as an illustrator is that you can choose to live somewhere with a lower cost of living because you can communicate and deliver finished art via the internet.