I'm of the school of thought that sits and works out potential sales, then I set an hourly rate of between £10 and £15 and depending on how long I've spent working on the product, I charge accordingly.
If I end up overcharging, I'll drop the price a little and see how much it affects sales (doesn't always increase them!).
If I end up overcharging, I'll drop the price a little and see how much it affects sales (doesn't always increase them!).