Saw this today on Tumblr!
http://artsplaining.tumblr.com/post/87214847198/attention-artists-need-help-calculating-your
It allows you to calculate how much time it takes you to do something and how much you should be making per the time you have spent on it. You can change the amount you wish to make every hour, just please don't undercharge yourself! :D
Link
Jacques Chat
I do my pricing by the size of the piece times a value for different media but I can see how this works for illustrators and other artists.