Under Dark Waves has almost as much content as the base game, it certainly deserves to be almost twice as expensive as Dead of Night. Honestly I find the prices kinda expensive, but acceptable, especially since we're unfortunately getting only one expansion per year (really wish they would come out more often). On the other hand, AH 3ed. is easily my favourite board game at the moment, so I'm definitely biased.

Yes, you can use those rules for any scenarios you wish, not only the expansion ones