Depends how much traffic the server gets (whether it's full 24/7 or not) also depends on your the net rate (how much date the server sends to the clients) before you can get that calculation.
It's not as easy as saying a 10 man server uses 10Gb a month, a 12 man server uses 12Gb a month, etc... A 10 man server, with a higher client net rate that's full 24/7 can use more bandwidth than a 12 man server, with a low client net rate that's full 24/7.
What connection speed do you have and how much bandwidth do you have a month?