For example, Mattermost's $10/user/month plan is proprietary software with roughly the features that Zulip provides as entirely FOSS (with a $3.50/user/month push notifications service).
By the way, since you mentioned Signal: Signal is great, but it's really just not comparable.
Signal is a SMS replacement/messenger app with minimal features that requires very low COGS per user, and launched with a $50M grant from a billionaire. Zulip is a team chat app designed to replace much more complex and capable products (Discord, Slack, Microsoft Teams).