I have no issue supporting Overkill with $10 Payday DLC. The free support they have given the game is amazing. They are on the forums always listening to community feedback like the Nation Red developers. I implore people to support them. Having said that i'm sure Amazon will the DLC cheaper than Steam ever would, if not at launch the shortly afterward.
There's that, which I do myself (see: Kingdoms of Amalur), but I'm speaking in general.
It's only logical. Push the main game in a sale and create demand for the new DLC.
Yes, but leaving it at full price isn't logical, especially in cases of games with many DLC packs, such as Magicka.