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...
If it was mid-2010, this was probably around the same time that S&W dropped M&P MSRP's by $40-$80 and more or less simultaneously stopped running the "$50 Rebate or Two Free Magazines" promotions on the pistols.
IIRC S&W had also run several $50-$75 rebate promotions on the P99 and PPS around that time, so perhaps this was an across-the-board change in marketing strategy!
FWIW P99's may have been $599 then, but IIRC the best price I ever saw on an M&P9/40 in 2010 was ~$500, and most stores wanted $525-$550. Now M&P's regularly sell for $475-$500 and go on sale for ~$450.