I think the price drop happened around the time of the release of the PPQ, in early 2011. Before then, the MSRP for the P99 was around $800.
Bought my P99AS in mid 2010 for $599. That seemed to be the going price for at least 6 months prior to that.
Not sure exactly when the price drop occured, but it was well before the PPQ was even announced.