There are many providers out there that allow you unlimited bandwidth. Why dont you shift to them ? Calculate the average size of your pages and then multiply that number by the number of hits you get. You should get a rough idea.
I've always gone with Startlogic.com myself, the price is right and they more than meet my needs. I've never had a super high traffic site before though, but one site I did maintain had a lot of graphics and I never went over the 50GB transfer.
The very existence of flamethrowers proves that at some time, some where, some place, someone once said to themselves "I'd really like to set those people on fire over there, but I just can't get close enough".