It isn't simple to nail down a firm price, but the single largest market place for PC guns is Gunbroker. The very TOP prices on PC guns come from the established, high feedback sellers that have a slew of great pictures and open their auctions at one cent, no reserve.

A 5" PPC-9 with original box, all papers and in high condition but not NIB/mint is likely going to sell no lower than around $2,800. High end would depend on who wants it that night. $3,500 isn't impossible.

Everything else being equal? 6" long slide higher, and not a little higher... significantly higher.

The 952 pistols always seem to be in demand the most, but the 952 was a catalog item. The PPC-9 in comparison was far more difficult to get.

As a side note...
I've watched these for a few years. All else being equal, a PC gun opened at one cent/no reserve gets WAY more interest than a similar offering at a high open price. And also... HIDDEN RESERVE auctions just squash interest. People hate them.