This is easy to answer: no! Your resulting Xbase++ applications will be royalty free. You can offer your Xbase++ application to thousands of customers without any additional costs.
For you as an Xbase++ developer, we recommend buying into one of our subscription plans: Foundation or Professional. These subscriptions guarantee 12 months of product updates, fixes and even new versions when available. Of course, you can upgrade the subscriptions by adding Technical Support and expect great help from our Xbase++ technicians.