This is sort of like asking "how much gas my car uses"
It all depends on the content of the site and the numbers of users. Bandwidth will not be much of an issue unless you have file downloads and then you can calculate filesize * users.
Determining and "average" is going to be very difficult if not impossible to determine.
You may want to concentrate your research on "server loads" as this is probably going to be a bigger issue.
http://moodle.org/mod/forum/discuss.php?d=50934 is a similar thread