To my mind the question would be "if an ISP has network with a fixed capacity, do their monthly operating costs differ between a month where they consistently utilize 50% of that capacity compared to another month where they utilize 80%?"
I suspect the cost to the ISP is the same in either case: they have purchased the equipment and bandwidth to support the network and those costs are fixed, regardless of how much of that bandwidth they are actually using. If that is true, this idea that there is only so much data per month to go around and if you use more then your neighbor then you should be charged more money would be fundamentally dishonest.