To be honest, you don't have to buy it since it's an DLC. Starting a post about the price won't change anything. I bought Dawnguard, I loved it and I think it was worth having. Skyrim is my favorite game because in my games I like to RP a lot. So with mods and extra content, Skyrim was the game for me and I'll keep buying any Skyrim DLC. Everytime I play the game with a new mod or in this case content, I feel like I'm playing a totally new game. The charm of Skyrim at it's best.
Do I think that price is steep? Yes. For most games, it would not be worth it. But Skyrim/Dawnguard is not most games.
As the user above well said. Skyrim really isn't like any other game. You get total freedom on a PC. Wish all games would go on with that path.