Welcome to the forum. Value depends on a lot of variables, but you've come to the right place to get honest advice.
When you say double dated do you mean it has the 1920 property mark and does it have a police sear safety ? Do the numbers all match ? Is the mag correct for the period ? When you say 60% is that just the finish, what kind of condition is the metal in, any unit markings ?
Sorry for all the questions but depending on a number of factors people might estimate from $500 to quite a bit more depending on the details, (a rare unit mark may add considerably to the value for example, severe pitting exactly the opposite !).
Many would say that any reasonable shootable Luger is now worth at least $500, (you can certainly find someone to pay that, anyway).
A few pictures will bring the estimates flooding in
Stu