purplewiz
Well, let's look at this item.
It was available for a limited time only, so it's not readily available. That means there's a limited supply out there.
There are only a couple of reasons items in limited supply go down in price: either they're re-released, which means the supply goes up, or they become markedly less popular than they've been.
The question then becomes: do you believe that either of these situations are likely?
Considering that HP only has one more movie and no more new matereal and the fandom is dying down (A major contrubutor to it's rise in value) I think it's a good shot they could become less popular.