I think anything beyond $8 would be robbery. Most services out there that are not online games cost around $8 a month, just look at XBOX live. It is not a direct comparison, but XBOX live is at least as useful as DDI. I think with the competition for net money DDI needs to stay about $8.
I really think you're failing to put it into the right perspective.
Firstly, it isn't the game. It's delivering new content and a database access to old content without having to own any of the freaking books. On that alone, it's amazing value for money.
Secondly, you're comparing apples and oranges. X-Box Live and WoW are completely different offerings. Both only network you with other people. Hell, you can run a WoW server of your own if you want, and not pay Blizzard a cent. The only reason you're subscribing is to join others who are playing at the same time. DDI is offering a huge amount of content, content which needs to be produced, play-tested, edited, etc. which takes a lot of creative energy and costs a lot more (comparatively) than networking a few servers.
Thirdly, there's the market to consider. WoW has 11 million subscribers. Blizzard could probably afford to drop their price to $5 a month and still make a killing. X-Box has a bajillion people as well, all pumping money into the system.
WotC is probably lucky if they have a couple of thousand subscribers to DDI. I would be willing to bet good money that they're currently running DDI at a massive loss.
Quite frankly, you should expect more from X-Box Live and WoW, based on the mere fact that there are a thousand times as many people contributing cash flow to it.