I don't think it has anything to do with the patch though. More like that more people played during the weekend and hence more competition on the market. Also, probably less buyers because people started alts for the double XP.
This is the most likely scenario: more supply and less demand results in depressed value.
On The Harbinger, prices are only down slightly. They usually float between 18k and 20k, just checked and the low price is 17099.