I agree with your analysis except that expecting 100% usage of the speed they sell you is a little unreasonable. All the ISP's VASTLY oversell the actual bandwidth they have. It is standard procedure to sell 100 customers on 10mbit internet and then join them all on a shared 100mbit pipe.  If the networks were actually capable of the full advertised capability 24/7 for everyone at the same time, a connection would cost way way more.  Understand that the max speed really is a burst speed that you might see sometimes.  If you look at the ISP fine print it usually says UP TO 24mbits.

This being said, the traffic limits are BULL. You are right, they really are robbing you. Even if you redo your calculation with an average of HALF the advertised burst speed, you are still getting screwed. Not getting the advertised speed is one thing, capping the total downloads this low is criminal.  I have two 60Mbit by 5Mbit charter cable connections in my house with two 680mhz DD-drwt gigabit routers and a dedicated server for downloading and a dedicated server for storage.  My data limit is 512 GB per month on a 60mbit connection. I tripled it one month easily. And yes I only have ONE CHOICE for internet.

I hope people start to understand how low these caps are and how sneeky the ISP's are getting.