If you pay, it's $10/user/month for the basic service. Where I live that's about 3x Ms Teams, 2x Slack. And you still have to self host.
Self hosting the service has the same issue as Rocketchat: you need an Apple dev account, a firebase account and an endless amount of free time. It's going to be far harder and more time consuming than just hosting the app.
I'm using it for a nonprofit, so $10/month per user is out of the question. Self-hosting and open source are critical to us also.