Lets take a look at Oblivion DLC for reference. There were several $10 DLCs and many of them were houses. You furbish them and that was it. Sure, they had special features for certain classes which made them worth it. So, with that in mind take a look at Fort Dawnguard. Your own room with safe storage containers, full smithing facilities including an enchanting altar, three salesmen, and all your vampire-slaying quests launch from that area. All without a loading screen. There's your $10 DLC right there.
So what about the other $10? Remember Knights of the Nine? I enjoyed that quest and found this one to be just as good. Whether it was traversing the Soul Cairn, hunting Elder Scrolls, or recruiting members for the Dawnguard.
Then we get extras: new weapons, spells, clothes, and much needed support for Vampires and Werewolves. Not to mention some interesting and rewarding side quests.
I have a feeling some people were expecting the self-sustainable exosphere of Shivering Isles. That was double the price of Dawnguard. I think this DLC is worth $20 easy. No more, no less.