Even though it may become complex I think it should be judged per project. WingIDE (Python IDE) for example gives out free copies of it's Professional version t

Even though it may become complex I think it should be judged per project. WingIDE (Python IDE) for example gives out free copies of it's Professional version to people who have existing semi-notable Python projects. So you could do something similar. Or perhaps just a discount. Just as long as people don't abuse your services.