If you are thinking or even just hoping to one day sell an image commercially, you should seriously think about getting this book:
Pricing Photography: The Complete Guide to Assignment & Stock Prices
It tells you what kinds of questions you need answered before you come up with a price for your image and then gives you an idea of what price range is expected given it's anticipated use. It also helps you avoid some legal pitfalls with wording of PO's, contracts, and the like.
(Buying the book via this link will give me a small commission and does not effect the price you will pay for it.)