300 € / year Fortinet firewall subscription with signature/IPS updates
Example M$ AD/Mail Infrastructure:
600 € Windows Server 2016 (Directory service, file/printer sharing)
You may want two because two Active Directory domain controllers can replicate, so your users can login when one DC is down.
30 € / Device/User that uses the Windows Server
580 € Exchange Server 2016 (Mail server)
You may use a backup Exchange server and/or external backup software.
40 € / Device/User that uses the Exchange Mail Server
10 users, 1 Firewall, 2 DCs, 1 Exchange (just license prices for servers and using them, NO client software like OS and office included):
Price: 300 € / year + 2180 € / infrastructure migration without having extra backup software, PBX, groupware, CRM...
If you use M$ Project or Remote Desktop Host Services you have the same server/CAL(client access license) licensing.
And there are virtualization licenses and so on... You need new licenses with new servers.
I know it's about saving money, but just to see what you get:
The "roles and features" of Windows Server 2016:
So my conclusio: You save money by basically using Nethserver and additionally with every service you use.