Provision a pro site by traffic

The features say you can allocate storage space to your pro site network but is there anyway to do it by number of unique visitors? Storage space doesn't really mean much to my target as I am having an S3 addon service anyway. Bandwidth is confusing to non-techies so I'd like to just say the number of uniques they are allowed but if thats impossible bandwidth would do. I need this as I am on metered bandwidth with my hosting arrangements.