It can be as low as €4/month + domain name, see:

https://docs.joinmastodon.org/user/run-your-own/#so-you-want-to-run-your-own-mastodon-server

From the top of my head, there are:

City of Amsterdam https://social.amsterdam.nl/
Baden-Württemberg https://bawü.social/ Netherlands https://social.overheid.nl/
Germany https://social.bund.de/ France https://social.numerique.gouv.fr/ EU Commission https://ec.social-network.europa.eu/

Switzerland had a pilot project, but I don’t know what happened to it

I’m pretty sure there are more local governments with their own instances or plans to do it , specially after all the digital sovereignty discourse got some boost to it