This is the wrong way to do the math. If HBO were to offer internet video they could choose 20 dollars a month and get maybe a million people, or they could choose 5 dollars a month and get 10 million subscribers. Which one do you think they will choose?
They could also charge 1000 dollars a month and get 1000 subscribers. At some point their cost/return normalizes and they maximize revenue. If I were to guess, it would be around the 5-10 dollar a month rate.
In any case there is likely millions of dollars a year left on the table that they ignore by not offering internet subscriptions.
If they were really smart, they would do something like this:
Game of Thrones $2 a month True Blood $2 a month Boardwalk Empire $2 a month Eastbound & Down $2 a month The Newsroom $2 a month
All current HBO series 7.50 a month All HBO access 10 a month.