I think it would be cool if someone could make a block that shows up while logged in as ADMIN but this block would show bandwidth consumption. Something checks how large every page is and it loads on everypage and does the math. Is this doable?
If you are using Firefox as your browser, you can download the web developer extension, and from the information menu, choose "view speed report." This will give you exactly what you are asking for, and more, for any Web page.