They are selling me 10GB a month but you say this is a variable feast.
Are they charging you for bits down the wire? Or payload bytes in TCP and UDP packets (ie not charging for transport protocol layer overheads, ICMP, ARP, etc etc)? Or payload traffic through their web (etc) proxies (ie only charging for stuff that the end user would recognise as data they've downloaded)? Or ...?
This could be a complete red herring, for all I know there's a standard way of accounting and charging and everybody does it the same way ... but if so I don't know what it is!!