I think most stats packages will give you an idea per domain. I know Urchin does for sure. But, these are measured in GB downloaded and if you have a lot of domains, its a PITA.

Most hosts bill based on 95th percentile in mbps (a running average clipping off the top 5% of spikes). If you have a dedicated server or multiple dedicated servers then I would absolutly insist on having an MRTG made available to you that shows your real throughput.