The PPK came out in 1929 and, since Hitler came to power in 1933, any "prewar" PPK is likely to therefore have at least some association with the Third Reich. The name stands for "Polizzei Pistole Kriminal" and was made for investigators and undercover personnel (Known under National Socialism as the Kriminalpolizie or Kripo) and of course was a popular choice for plainclothes agents of the Gestapo.
Most PPKs manufactured after 1933 onward to 1945 carry various Nazi proof marks. Those with Gestapo markings fetch very high prices, and those that come with documentation that they once belonged to Nazi functionaries can go for tens or hundreds of thousands of dollars. The last estimate I saw on Albert Speer's PPK was $750,000.
Prewar PPKs without any Nazi marks are accordingly less common, but they are less in demand; and if you have ever been at a gun show you know that there are some people who would pay a hundred bucks for a bag of dog poop if it had a swastika on it. (really. $150 for a cheap copy of an SS dagger with a thin sheet-metal blade? LMAO!!)
In any case, a prewar PPK in mint condition without any Nazi marks will probably be worth between $750 to $1,200. But that's just my estimate. The problem with buying "collector" or "period" firearms is that an appraiser may say one thing, but the people who actually want t posess the weapon may say quite another. I bought a S&W vintage K-38 Combat Masterpiece (Pre-Model 15, made in 1950 in original box with tools) for $950 two years ago. Various appraisers quoted me from $600-700; but the fact is I was accosted four or five times as I walked through the gun show by guys offering me $1,100 to $1,500 for it.
In the end, what you are looking at is worth whatever you are willing to pay for it. Collectables are a funny thing.