Hi,
1) It is true, Azure charges you by the hours you left Ant Media Server open, user count and data transfer we don't earn anything from that area but Azure may charge data transfer which is not related with us.
2) You don't require a STUN or TURN server, Ant Media uses Google's open STUN server without any problem. If you have customers from company firewalls or intranet, depending on the structure you should either use TURN server or you can stream with DASH or HLS from the 80 and 443 ports which must be open to reach the internet.
Cheers.