All servers economy should be dipping at the moment. In my opinion it has to do with the recent addition of augmented items.
Every time someone creates an augmented item from a previously regular AH-ready item, Gil is being destroyed from the server.
Hundreds of people on each server are taking items (maybe just as small as 5-10k or even as expensive as a Penance Robe) and immediately turning it into an item worth nothing more than what an NPC will pay for it.
I'm sure the initial surge of augmenting is over for each server but I can still see this effecting the economy for a long while to come.
This wiki page is a really good read for anyone who uses the AH:
http://wiki.ffxiclopedia.org/wiki/How_to_be_good_for_the_Economy_-_Guide_by_ChapelIt explains how gil being created and destroyed can effect the server. It's long, but to sum it up:
-Creating gil (Quests, Missions, NMs, Mobs, crafting at a profit) will increase the amount of gil on the server.
-Destroying gil (trading items to NPCs, augmenting items, throwing items away, crafting at a loss) will decrease the amount of gil on the server.
There are hundreds of ways to effect the economy, and i'm not saying that this is the sole cause of the drop in prices, but destroying gil faster than it can be created is a great way to effect prices.
Just my two canadian cents :)