Some hints on setting up your own shoutcast server
The shoutcast server will not waste bandwidth when no one is listening. Just the brief connection to Shoutcast's own servers for the directory listing. But that is almost equal to zero.
You need to bring up your calculator to get the monthly bandwidth usage. Say you have set your bitrate to 64kbits. That is what it takes for one person to listen per second. That is 8KB/s and user. then multiply that with the number of listeners and the time. You quickly reach quite large numbers even for low bandwidths.
You cannot have higher bitrate than your own uplink. Minus overhead and stuff like other usage of the internet connection. Say if you have 512kbits uplink you can only have 7 slots of 64kbits. (that is 448kbits) and you'll have some bandwidth for your self other things like surfing the web. I'd go for AAC+ format instead of MP3. It uses less bandwidth but provides better quality. My 48kbit AAC+ sounds quite near the 128kbit MP3-sream.