The true cost of expending any resource that has alternate uses is the foregone opportunities that those alternate uses represented. When asking whether the DLC is worth 20 bucks it's certainly relevant to examine what else could be obtained with that 20 bucks instead, including older but as yet unplayed games.
Whether Dawnguard is worth twenty bucks will vary incrementally from person to person based on the scarcity of both money and the time available for playing games weighed against alternate interests. For me it's not currently worth it because the competition for my time from already purchased other games (go Steam Summer Sale!) significantly reduces it's value. Had it been released a month earlier, before the sale, I would have bought it on release.
I'll see your rational actor and raise you a horde of people with poor impulse control and an angry mob.

Anyway, I don't disagree at all that timing and other factors are part of the buying choice. But BGS is following a standard, simple rule that every media industry uses: start out selling high, then drop the price when interest drops. Rinse and repeat.
What I was talking about earlier was the issue of calling a price (of a new release) unfair because prices for old releases were cheaper. While technically it might be a more logical choice to wait to purchase everything until a combined GOTY or whatever is released, there's nothing unreasonable or unusual about a company selling something at the highest price they think they can get away with. Of course, with the contemporary corporate love for risk mitigation, you could argue the opposite, too. There are any number of "logical" reasons to rush buy a product now to mitigate the risk of deferred gratification.