The terms of a license can dictate how much you should charge for something. For example, consider:
- Am I selling ownership of this image, so I can no longer use it myself?
- Am I only selling usage rights, so that I can keep ownership?
- What are the terms of the usage rights, how long do they have usage rights, where, and how much money will they be making?
Search the web for up-to-date information, ask fellow illustrators what they consider when pricing artwork, and check out these resources: