HBO as an add-on to cable packages costs ~$20. If HBO were to offer it as a standalone service, they would have to handle billing, subscription management, increased marketing budget to make up for marketing that cable/sat currently offers for them, etc. So unless a critical mass of people are willing to pay AT LEAST $30 per month, this is a lost cause.
HBO currently gets around $8 per subscriber, typically you are paying double that to the cable company so unless the overhead is 100% I doubt they would need to charge more than $15 per subscriber. The real problem is distribution and making up for the fact that cable companies are guaranteeing a certain number of subscribers (in the millions).
The other reason is that Time Warner wont risk their other channels being unbundled from the basic tier such as TNT, TBS, CNN, Cartoon, etc. They make more on some of those channels (Ads + subscriber fee) than HBO.
As several people have said, Netflix or Hulu would probably be happy to do the billing and management. The marketing is definitely an issue, since cable companies can efficiently market HBO to existing cable subscribers.
HBO's cut is very small (< $3 I think). But the cable and satellite cos pay a fixed fee per base cable/sat sub to HBO but then upsell their own customers and recoup that cost. Using the $20 as a proxy for about what HBO gets, even though it's a lot more complicated than that in reality.
