Let's assume they have M365 E5 at $57/m/user. A small government is several hundred people let's use 300.
300€5712 is a yearly cost of ~34K
E5 license includes
Office 365. That can be replaced with Open/Libre Office at minimal cost.
Teams unified communications suite. You would have to go Slack/Zoom combo to get the same capabilities at a monthly cost per user for each.
SharePoint/OneDrive. Not sure of Linux versions.
Email with anti spam filtering. Postfix with MTA that filters maybe.