Speaking for Zulip, in this kind of situation we are happy to work out discounted Zulip Cloud pricing that makes economic sense for what you're using it for.
Our take is that the full price (which is similar to Slack's) is designed for when the users are full-time employees that you're paying a rich-world salary, and that in that situation it's very cheap.
Here's what we say on our pricing page: https://zulipchat.com/plans/
> Zulip Cloud Standard is free for open source projects. We also offer steep discounts (usually 85%-100%) to many non-profits, educational institutions, groups of friends, and in other scenarios where most of the users are not fulltime employees of the customer. Generally, only closed organizations that also pay their members' salaries pay full price.
