This question came up in another recent interview for my book-in-progress about creatives and money. I was talking with Monique Elwell, CEO of social media marketing start up, Conversify, who will be leading a breakfast roundtable at the next Creative Freelancer Conference (June 5-6 in Denver CO). Monique is also the business partner of Aliza Sherman, who will be speaking about online marketing at CFC.
Monique's short answer: you are probably not charging enough.
Listen to this 2 minute clip to hear Monique's take on how to figure out what to charge.