Not worth it........
Between the normal modem/head end chatter making it just too hard to measure, and page adspace (much of it high bitrate) users don't want, ask for, and have no control over, measuring down to the byte just isn't worth the trouble.
ISP's could create multi-GB zones, by which could more readily be measured, and bill users according to the 'zone' their usage has landed them in. I realize there would have to be a fine line between these 'zones', but it wouldn't seem to be nearly as messy a billing by the byte.
Just a thought.