I think $12.50 is perfectly acceptable for the second largest and likely most desirable premium network out there.
I'd expect that most folks probably watch fewer than a dozen channels with any regularity. Nielsen says that the average household was receiving 130 channels yet only watching 18 of them in 2008 [1].
I think we eventually get to an a-la-carte system with app-like monthly price points where premium content falls comes in at $5, $10 or more and there are dozens of solid options in the $1-3 range with loads of free (ad-supported) content.
Really? I think the price is great, assuming it's a high quality stream with no ads, new episodes are made available promptly, and you intend to watch at least two or three HBO shows.
HBO's typically ~$7-10 on top of your normal cable bill anyway. If the regular networks went down this model, you'd probably be buying all of [Time Warner, NBC, Disney, Fox]'s channels at $10/mo rather than one by one.
I watch quite a few shows across different networks. I guess this is a good start though. If they did follow with options similar to posted in the previous comments then I feel it would be superb... Though I feel like these days it rarely happens.