In a recent Wall Street Journal article (the same one several of us have been quoting lately in regards to how many coins TPGs grade per day) they discussed how the very top end of the coin market (coins that sell for six or seven figures) is booming but the rest of the market is in a recession. In short the reason is that the really rich people are still doing fine while everyone else has to watch their non-essential purchases.
So my guess is that less people are in the market for coins like yours (not high end, but not cheap either), so when one goes up for auction you get less bidders, and less bidders means a lower sale price. Although the price of gold does have an effect on the price of a coin, it there isn't a direct 1 to 1 relationship. Remember that the numismatic value of the coin is what makes it worth more than purely the amount of gold in it. While the gold value of the coin may be going up, the numismatic value of the coin is going down to a larger degree.