The first problem is that you're conflating volumes of data and transmission speed. I'm pretty sure if your claim of "100 times more bandwidth" made any sense, it would mean you're assuming your customers would be perfectly happy browsing websites over broadband technology at a dialup speeds.
You clearly do not understand transit and peering agreements. Typically, Level3 should be charging Comcast extra because Comcast is sucking down traffic so asymmetrically. This is happening because Comcast has been attempting to engineer their infrastructure to be as lopsided as possible--mainly because they're cheap assholes. If, on the other hand, they had customers who were actually allowed to produce content, Level3 would be obligated to be less snippy about the volume of traffic Comcast is "consuming" because peering agreements are about simply saying "We could charge each other money, but we're both using about the same amount of each other's traffic so let's just call it even".
To make it clear where you are misunderstanding how all this works, there are no per-byte costs for the transmission of data when you own the hardware. There are minutely costs of maintenance, and minutely costs for operation (which is largely electricity), and upgrades should be budgeted with the same approach. Period. In fact, if you do not know approximately when your next upgrades will be and what type of upgrades they are, you have already failed.
You've either purchased sufficiently capable routers or you haven't. It only makes sense to start getting picky about the volumes of traffic when your hardware is insufficient to the workload presented, which means it's either already past time for you to upgrade or you did not deploy sufficient equipment from the start. Do not make the mistake so many lazy-assed cable companies are making and assuming your infrastructure is something you deploy once and profit from forever. Budget regular upgrades and include them in the costs to the customer or stop pretending you know what you are doing. It will not matter if your competition (if it even exists) is charging a bit less when their service will be unreliable and slow because they did not budget properly.
If you are in a position of management for this ISP and do not understand these things, then you should get out of the residential internet business now and let someone else who knows what they're doing handle it.