What do you plan to host with the shoutcast? Audio is a lot less bandwidth intensive than video.
I used to host shoutcasts, I had 1 video and a few audio. The audio shoutcasts were almost never full, or even close to it. So they could be offered for a decent price. But video used gigabytes an hour, and couldn't be maintained cheaply.
Really though, it depends on how you encode what you broadcast, and how many listeners there are. If you offer someone 128k streams, you can count on them using the 128k. But then their server probably won't be full.
I recommend running your own shoutcast servers for like a week to help you get a feel for it.
I've gotten rid of my old way of approaching it. I used to say "you can broadcast at this bitrate with this many listeners". But its harder to price for that. So I allow users to run them in their shell accounts, set restrictions so they can only run servers on their ip address, and then their total bandwidth is measured as the traffic on their dedicated IP. So I can say "you have 150gb of bandwidth to use". They can use it for a shoutcast with 900 listeners for an hour, or 20 listeners for a month, or on a website... how they use the bandwidth is up to them.
In my experience...
Yes, it uses a lot of bandwidth, especially if they're full.
Shoutcast servers put little load on the server's hardware itself.
There may also be legal issues to consider if your users are streaming mp3's that they don't hold the copyright's to even if they were downloaded / obtained legally it still dosn't give them the right to broadcast them, you probably need to talk to your legal counsel concerning this.