1/3 the cost of the original game? Using resources already available in the game? Oh, had ther better be more than a couple hours content for that kind of money.
First, as others have already said, it's supposed to be more of an 'expansion' than Fallouts tiny 'DLC'.
Second, that's actually a great deal considering how many hours of play we'll probably get out of it. Skyrim was already rediculously underprices at $60. Most companies would give us '5 new maps' that I could easily design in a month for $5 or $10 that use the same assets as before, maybe one or two unique set pieces. This DLC has new factions, story, dialogue, places, people, items, abilities, and the list goes on. I'm very pleased that it's
only $20.