We are a company with around 100-150 employees, using Mattermost in a real Enterprise setting for years now (with SSO, CI and mail integration and whatnot) and that server is being maintained by our admin team basically "on the side" - nobody was hired specifically for this job, as it does take less than 20% of one person's time according to what the team says.

These Bay Area tech people must be either incredibly inefficient (especially when comparing to what they get paid), or your estimate must be waaaaay off.