I, like so many others, am still trying to make a splash (or at least a ripple) in freelance illustration and writing. Here are some pages I found helpful, and hope you will too.
Payscale.com - Calculate how much your time, skill, and education are worth.
How to Charge - Should you charge by the project or by the hour? This article gives you an idea.
FreelanceSwitch - Hourly rate calculator. This actually tallies up your costs and gives you a more specific number to put next to the dollar sign. (will take 5-20 minutes to complete)