In the end, a firearm (or anything) is worth what you are comfortable spending and are satisfied with. $1200 isn't too out of line, though I think he was either shining you on, or the "collectors" he was talking to were out to help him buffer his wallet. Regardless, hang on to it (or have Dad hang on to it) and it will very soon be worth that $1600 if recent history is any example.
There is always "Asking" and "Selling" prices, and the two are seldom the same...
I see a lot of firearms collectors frequently "asking" about twice what a like weapon is currently selling for...speculating, holding on to a "dear" piece, or waiting for that sucker who "must have it now!"
Did the "Collectors" he was refering to work at Simpsons?
Congratulations. Now that you've bought Dad's Luger, when are you buying your own? May I suggest a WW1 piece to augment your collection. How about a nice Artillery? That'll show Dad!!!