DVD boxes of dramas are very expensive in general in Japan. A box set of a regular drama (mostly between nine and twelve episodes) usually costs $200 and more. As for why they are so expensive, I really have no idea. I personally think they're really not worth the money because let's face it, how many times are you probably going to watch one and the same drama... And they never carry subtitles so for people who can't understand Japanese it's so not worth it even more.