Typical bandwidth usage

I have a strange question: about a month and a half ago we launched a new 
site for a client that has been very successful. As a result, their hosting 
provider is dinging them with a massive rate increase for bandwidth usage. 
The IT manager is trying to pin that on us for some dysfunctional reason 
rather than talk to their ad sales department about increasing the budget 
allocated for hosting :(.

Without going into the corporate structure of this particular client, I do 
think we need to protect ourselves with some statistics showing that the 
page weights are normal, and that they are simply victims of their own 
success. To do that I need to find some stats on typical bandwidth usage 
per session/day for typical websites. Any links would be appreciated.

Secondly: they are experiencing in the region of 40,000 page views per day 
and their season is still warming up. In the opinions of the web server 
professionals on this list, what would be a typical bandwidth (Mb/day) use 
for a professionally developed site with that kind of traffic? 
Unfortunately I cannot post the URL, but suffice to say that we design 
within the same guidelines as most successful professional web developers.

TIA
Cheers

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