AFAIK you can run your own ZeroTier controller for free. It's just not documented too well and also it's missing the web UI for managing your networks.
I ended up writing a CLI to do it that's relatively full-featured. At some point, I intend to move functionality to a shared library between a CLI and a Web frontend, but for now, the CLI works tremendously well for my use cases:
They charge for management of large networks on their controller.
You can run your own controller fir free, and it's also open source iirc, but it's not as nice (no web ui) and you're fully on your own.
It's $100/month for licensing our web interface for controller management. You're free to set up your own network controller and write your interface for managing it :)
Ah, excellent! I have only used zerotier with them hosting. Would far prefer to self-host. Thanks for pointing that out!