I'd suspect Beth won't put it on sale until they can figure out can they release Dawnguard for PS3.
This.
As outraged as the PS3 fans are, there's quite a lot of them still shouting "shutup and take my money."
But yeah I'm in the same boat. I am NOT paying €20 for....well, any DLC really. Dawnguard also looks particularly bad in providing new content. As I've said before, it looks as though they took a TON of stuff from Gamejam, copy-pasted it to Dawnguard, add some crossbows and a quest chain and violá. I'm sorry, but that doesn't seem like a whole lot of content considering half of that content existed pre-Skyrim release. And even if the DLC did have as much content as it should (for me, the New Vegas DLCs did this (Point Lookout also deserves mention)), no, I'm not paying twice what I used to pay. I'm not supporting an increased price tag just because you think you can get away with it.
If we're to be really critical here....
New Vegas had one year development time (around that) and was $50 (60?) bucks. Every 3 months, a DLC was released for $10. So basically, they took 1/4th of the time of the base game and costed 1/5th of the price. This is fair. If Bethesda asked for $12.50 for the DLCs instead of 10? Sure, that's fair too.
And no I'm not saying development time should have a DIRECT correlation to price, was simply an example of the amount of effort I expect to see put into it. The price doesn't have to correlate with the base game price, the quality
DOES. For example Oblivion took 4 years to develop, Shivering Isles took one. Oblivion was $50, Shivering Isles was $30. Still worth it, still fair. Why? Because Shivering Isles was good. It was a high-quality release that was dramatically different from the base game, added all sorts of new stuff and was simply fun and made with love. You know what else did this? The New Vegas DLCs. Dramatically different? Check. Added bunch of new stuff? Not as much as Shivering Isles, but for their reduced pricetag yes, so check. Fun and made with love? Oh yes.
Unfortunately, Shivering Isles was an expansion; now we're talking DLC, and a lot of companies seem to consider DLC to be "more of the same." If it truly IS gonna be more of the same? Not a dime over $10. I'm also not paying $5 for a damned house mod (that I can get for free on Skyrim Nexus...).
Dawnguard doesn't seem dramatically different, it's more of the same but with vampires tossed in. Dawnguard doesn't add tons of new stuff, it adds vampires, crossbows, and a bunch of enemies that got painted black. Dawnguard doesn't seem to be made with love, no, it seems to be made because a developer was a little too obsessed with that Underworld TV show and wanted to make his own fanfic.