But I'd be much faster to pay 15-20 than 65+(15 to 20)=80+ and watch 5 channels + HBO.

I think cable misses an opportunity by forcing you to have a heap of channels you don't want just to get the stuff you do. I'd pay somewhere around $20 a month to get the probably 5 or so channels of any interest.

So.. what opportunity are they missing? The opportunity to get less money from their customers? Doesn't sound like much of an opportunity to me.

The opportunity is that they could have more customers. Many people are canceling their cable subscriptions because the price is too high for the value it offers. If cable companies allowed customers to have only a few channels that they cared about for a lower fee, many of these people would keep some level of cable subscription.