Packet information the game sends is small, but there are a lot of them. Even so I don't think you could hit 300 gig in a month if you played non-stop. Send Skuzzy a note and he should be able to give you some estimates on file size. Remember there are players who use satellite connections with charges for data and they are still playing so the data exchange cant be all that big.
As for charging you what they want, ummmm thats how people make money. It may be time to hunt around for another service provider. There is no better feeling in the world than when you get to call a big company like Comcast and tell them to stuff it! 
Getting a new provider is easier said than done. I live in NYC, you would think that I would have my choice of TWC, Verizon, RCN, Optimum...
but NO! These companies are in control of well defined, territorial monopolies, most peeps do not have any options but 1 or none. Instead of the government forcing 1 internet hardware design that all providers must build, they divied up the landscape into little fiefdoms that they only control and only provide their hardware and infrastructure.
Because of how the internet infrastructure is so piecemeal, they got us by the balls, in a monopolistic catastrophe that is totally against the free markets! I now pay $50+/mth for DSL crap service, raised my rate without even a mention. I can get TWC 50/5 service for same price, but I already thru TWC out for erroneous billing and bad service. So now I have to go back to them or have none.
So you are correct that they can bill what they want but in a monopoly its BS.
Good luck OP on your dilemma, I feel your pain. I wonder if they charge the government for excessive use by the various security agencies.................
