The only settings I can find are for limiting in terms of bytes per second but if I understand correctly that'll mean that the connection speed of the site is being throttled as well. What I want to do is to have a hard limit - say 30GB/month, but I don't want to limit the connection speed. Thanks.
That option doesn't exist in IIS. If you had a have a hard limit, you would essentially have the site stop X days into the month once they reached the limit.
To accomplish this, you would need to use an external tool to record the bandwidth per month (whether it's a statistics program, or one that records bandwidth in real-time on the way through). Then when the limit is reached, use one of the IIS APIs to stop the app pool.