I never bought a DVD of an Anime. They are just too expensive! One DVD of a good Anime costs about 40-50€, and its only the first Volume with 1-5 Episoded
If you accept a waste of time too, I'd say Hyakko. Dunno what I expected when I first began watching, but boy, do I regret it.
In terms of actual wasted money, nothing really.
Looking back on it I could have gone without buying as many Bleach volumes as I did and a copy of Akira Club even though the problems I have with that book are Amazon's fault.
detective conan for me. I bought far too many of those back in the day and i don´t really plan on rereading them anytime soon. Haven`t really bought a lot of physical manga/anime since then.