These pistols are uncommonly encountered but probably not "rare".
Schwing's Standard Catalog of Military Firearms lists a pre-war/wartime .22 PP in VG condition at $600 and Exc at $800. The 28th Edition of the Blue Book of Gun Values lists a Nazi-proofed .22 PP at $675 for 90% condition, $800 for 95%, and $975 for 98%. Add another $100 if you have the original holster in decent condition.
Sounds like $600-$700 ought to be about right for just the pistol, but ultimately the market will determine the true value. Listing it on one of the auction sites will probably net you the most money.
-Lee