The software itself is 100% open-source, which means there are zero license fees forever.
The “costs” you will run into are purely related to the infrastructure needed to run it—essentially the server, storage, and the support to keep it running.
If you prefer a more stable environment, you can use a managed cloud platform like AccuWeb.Cloud. In this case, you only pay for the actual resources your instance consumes, such as RAM and CPU.