I have been using a free plan for over two years. Zulip is awesome and, in my opinion, way better that slack. I use both on a daily basis.
We run our zulip on a smallish EC2 instance. It's probably less than a Euro/month/employee.
I have had pushback from non-technical staff (and pushback from tech workers for a week, but now they seem to be very happy aside from UX bugs esp with the mobile app).
If you're ever looking for work, we are hiring across the board, fully remote in Sweden or Germany. (due to tax!)
Thanks for the hint. As a German speaking Swedish that could be a great fit. I have coded a component for Lamborghini but not a part of my career I am particularly proud of. There should be smarter things to do than ruining the planet. So don't expect an application despite of zulip ;)
Hopefully my sim racing game that I am building will be good enough that we can stop wasting so much fuel on real cars. :)