The price of ore, as with other materials, depends on how much is being harvested for sale and how many buyers there are. On my server - Aerie Peak (EU) - there's close to the full complement of people and yet the prices vary, sometimes wildly.
For example, Iron Ore was going for 9g a stack a few weeks ago. Now it's down below 7g because less people are buying or more people are selling or a combination of the two. Also, Huge Emeralds, the gem of choice for the 290-300 jewelcrafting were hovering around 9g each just 10 days ago, when I was grinding through that period. The weekend just gone, they were down to 5g and now they are back up to 8, which is fortunate for me considering I bought 16 at the lowest prices.