Chinese have found a new way to dupe shit. This happened roughly 2 weeks ago. And this is also why 15-20 days ago prices were still very high (0.80-1$/k or even more) and then they crashed.
Not only chinese are duping shit actually. There was a post on Ownedcore elite section a few days ago where people were discussing about a new working gold-duping method.
Prices will definitely drop more and more until Blizzard finds a way to fix the duping. Maybe with the forthcoming 5.1 patch.
To conclude, price dropping was brought about by duping and only by it.