I'm sure there is a solution you can run from your web server. What OS is it running?

It'll take up as much bandwidth as the stream per user. If you're running a 128kbps stream, it'll use 128 kilobits per second (plus a little TCP/IP overhead) for each user listening.

There are services, though, that'll allow you to stream to them and they'll multiply it, though, so you only use up one stream's worth of bandwidth, and your users go to their server to listen. But all of them will cost actual money, as far as I know. Live365 comes to mind.