Around $50 for a B4ms in Azure, automated to shut down overnight, etc. Enough for around ten people to use simultaneously.
For instance, the server at <https://galene.org:8443> is running on a "Value" OVH VPS (<https://www.ovhcloud.com/en/vps/>) which is rented for under 6€/month. During lockdown, It was regularly used for lectures with 70 students or so, but that's an unfair comparison, since students tend to keep their camera off during the lecture.