Zulip Business costs $6.67/user/month. While the only Zulip feature that can't work without purchasing a plan (or doing a huge amount of work to publish your own mobile apps) is mobile push notifications for businesses with 10+ users, https://zulip.com/plans/#self-hosted details dozens of features for which Zulip Business includes expert support.
Zulip is 100% open source. In open core products, those specific features just don't exist at all in the open source version. For example, Mattermost requires the proprietary Mattermost Professional [1] at $10/user/month if you want to use SSO, LDAP sync, user groups, read receipts, etc.
(Note also that $10/user/month is the minimum price at which Mattermost offers push notifications "for use in production environments" [2].)
If you compare Zulip's pricing options to Mattermost's, our offering is substantially more generous. It would be even if we went open core and moved a few dozen features of Zulip to a proprietary version. But we're not going to do that: we really do care about being an 100% open source project.
If you instead compare Zulip Business to using Zulip for your business's mission-critical communications between paid team members without paying for something, I think that's the wrong question. Skilled humans' time is incredibly valuable, and people spend a LOT of time in team chat tools. It is rational for businesses to pay a tiny fraction of the fully-loaded cost of their employees for the insurance that comes with support and mobile push notifications for their main collaboration tool.
(If you're using Zulip for something other than a server full of paid employees and contractors, see the Community plan and discounts for other use cases).
Finally, on the topic of cost to us: Supporting a business that is self-hosting complex, mission-critical software like Zulip is more expensive for us than having that business using Zulip Cloud. Hosting is cheap compared to humans who can debug anything, and there are big economies of scales in terms of human time for managing a large multi-tenant cloud installation.
Helping thousands of different people with varied skill levels self-hosted your application successfully is not cheap, even for a project like Zulip that is very focused on making self-hosting Just Work.
[1] https://mattermost.com/pricing/features/ [2] https://docs.mattermost.com/deploy/mobile-hpns.html