I don't think there has been a decline. These are still fetching a premium if you have/want a good quality one. Individual prices may fluctuate, but the price trend on most sales is usually still high.
There's been a decline from the prices at the start of 2011 for most MCC. However most of us were not stacking pandas back then so probably didn't notice that much. It's not just the mirrored Panda, other coins have fallen from their highs including but not restricted to: Unicorns, Pagodas & God of War. That is not to say the mirrored is a bad buy, in fact it's an amazing bargain right now! The panda/MCC market seems to have finished it's correction (I am guessing only) in the short term and is looking bullish. I recall snagging quite a few bargains such as the 2oz Silver 1991 Piedfort Panda for $800, you can't find that below $1000 nowadays unless you hunt hard. Another reason for the decline you may see going on right now with some coins is because sellers are trying to raise cash now so they can pay their tax due in April (US).
In pricepedia abril_may 2011 the mirror are at 3000 (now 1100/1200) and the frosted for 600. (now 400)
I have one of these available for sale now in NGC MS69. Also have the Frosted in NCS MS69. Need to sell, so if anyone is interested just shoot me a PM. Thanks all.