What the market is saying (at least on Amazon) is that MoP is not worth the release price. Market demand has lowered so much that the price has to be dropped to move the product. The price is going down because the market says the "bank for their buck" is much lower than the release price.
Products don't drop in price because people are buying more of it.
That you think an expansion who's price has nose-dived after three months means it is "doing better" is so silly that you are even having a real discussion anymore.
Makebs price has "nose dived" the moment it appeared for pre order by 50% by your logic. Nice one.
It also doesn't have 3 new operations 6 new flashpoints 2 new warzones a new race and a new class. I don't know how any one can say this cruddy little swtor expac is in any way shape or form up to the standards of a real MMO expac.
Or 6 new planets (zones).