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.
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.