Not affiliated with Zulip, but have you checked the "Free and discounted Zulip Cloud Standard" info on their help center? https://zulip.com/help/zulip-cloud-billing#free-and-discount...

Sounds like you could be eligible for free or for a significant discount. Also:

"If there are any circumstances that make regular pricing unaffordable for your organization, contact sales@zulip.com to discuss your situation."