Well, it has advanced quite quickly imho. This is due to the players knowing what they are doing, knowing how to bring gold into the game.. you will see a slight decline in gold prices. When they first came out 1m was over $50.. Now it's at around $4.50/M. It's been quite stable for the last month or 2, but will decline shortly.
The more money in the game, the easier it is to obtain, therefore the prices will adjust accordingly.