I picked up some extra copies like 3-5 years ago of some limited edition DVDs that I hoped would increase in value. Some that did:
Last House on Dead End St. (wow! going for $300 new)
Let Sleeping Corpses Lie (over $120 new on amazon)

But I bought a shit load of the 2 disc Army of Darkness DVDs (well 5 of them) and they seem to be worth LESS than the $50 I paid for them.

Same with Halloween 2 disc collectors edition.