There are quite a few factors involved in the field of pay in illustration.
1)The company: Pay varies drastically from company to company, and not just the scale. Unless you have worked with a company for a length of time, be wary about receiving a check (and even then, there are companies that duck out of obligations). Art (exterior and interior) often sell products, but paying illustrators isn't always high on the priority list.
2)The market: With the d20 event, work is much more prevalent in the rpg industry, obviously, but the ccg market is usually much more lucrative and fairly steady if the game is a good seller. I know people who have bought houses off of checks from ccg art, and people who work a square job and spend their spare time working for rpg companies.
3)Factor X: The ability to deliver quality on time is very crucial at the beginning stages. It is always of utmost importance to be able to understand how to create concepts, how to work neat and on time and to be sure of your own limitations. This is a field that is lucrative if you are good and able to adapt, for example, why work exclusively in the rpg market? You will find that many, many artists and illustrators work in various fields in the same or similar genres. And luck and timing never hurt. The money is there, but you have to think of your own long term goals and dreams as well.
hellbender