As you no-doubt know, prohibited firearms have very limited value, and their value decreases over time - which is the opposite of collectable firearms which are restricted or non-restricted. While things like K98k's, Garands, HK P7's, restricted Colt Pythons, etc. climb to unheard of levels, I would venture to say that 12.6 prohib pistols have signifigantly decreased in value since the 1990s - for the simple reason alluded to above - the people who can acquire 12.6 pistols have been able to do so for 20+ years, and they generally have long since acquired whatever they want in this category. The pool of potential buyers is small (compared to restricted firearms) and is getting smaller all the time. Eventually, if there is no substantial reform to the Firearms Act, people will hardly be able to give away prohibs. I think this is already happening in terms of some of the prohib rifle classes (an acquaintance of mine told me how there was a converted-auto M14 reciever sitting in his local shop for a couple of years. Eventually they just gave it to him, as the pool of people who were interested in it + had the licence to acquire it was... just him. Locally at least).
My point: don't pay very much for that PPK no matter how nice it is. Unless you are planning to export it to somewhere where the firearms laws don't cripple the value of these types of firearms, it has little to no value anyway. A token amount like $400 is what they go for.