The Walther PP was made for many decades, with different editions and places of manufacture made over those years, so that is probably the reason that the prices of these pistols can be up or down, depending on the year of manufacture or condition of the pistol.
I would also try posting this question on Waltherforums, on the "PP and TP series" section, as I've noticed that the guys there really know their stuff when it comes to Walther's PP series pistols.
A few years ago at a gunshow, I ran across a 7.65 (.32) Walther PP that looked like it was brand new in box, with a deep blue finish that looked as good as an older S&W revolver, for $400. I'm still kicking myself for not buying it.