Advice: Extra Charge for Commercial Use

[sambeawesome.livejournal.com]

I know it's typically standard to charge extra if someone wants to use your work commercially and the price varies based on what they want to use it for, how long, etc. I just have no idea what to charge. I know it's hard to give ballpark estimates without specifics, but maybe some ideas of figuring out the math?

Any outside references/resources would be super helpful too. I want to start branching out more with commercial stuff and I'm a huge newb at the moment.

Any help/advice is appreciate though!