I'm just starting a server on my aDSL connection. I have
www.netlimiter.com installed to throttle bandwidth. Of course I haven't checked it with players other than me, but I was showing very little usage at all. I was really surprised.
Also, remember that 8 bits = 1 Byte so...
12,960 Mb/month = 1,620 MB/month = ~1.58 GB/month