You reminded me of some WWII items my Dad left me. He was with the 101st at Bastogne and got a P38 and holster among other items. What is a P38 worth and is a actual battle won pistol worth more?
The P-38 was actually a better sidearm than the P-08 Parabellum, in my opinion, although both exhibit the same fine craftsmanship common to German weapons.
Take your piece -- with the holster and whatever documentation you can put together -- to a gun show and ask around. There are collectors there who are experts.