Dawnguard isn't having a retail disc option for either PC, Xbox360 or PS3. This cuts back on the cost considerably, particularly to the publisher (Who form the bulk of the markup for profit).
While I'm not saying this guarantees Dawnguard will have the same content level of Shivering isles, price alone cannot be used to make this call. The market has changed so much since Shivering Isles. I think we're definitely looking at something larger than Point Lookout or Broken Steel though.
It's been said, and unfortunately it seems its going have to be said another time.
Todd had announced before the game even came out that all the DLC would be smaller than SI but bigger then Fallout's DLC's. Now thats been sorted i see no need whatsoever for these types of threads...
These were their pre-release plans. When Skyrim shipped 7million units in the first 2 days of release, I think they'd be stupid not to at least reinvestigate the possibility of scaling their post-release support up quite a bit, including DLC.
We've actually already seen it in the form of patches with not just bug fixes, but new features as well.