What would a used 9mm Luger worth?

There is no one answer, as there is no one Luger pistol. There are more than 100 variations of the Luger pistol- some relatively common, some extremely rare. Value would also be driven by condition, and if it has had any changes made to it. Value might be $600 or $6,000. You will need to take this to an experienced dealer who can give you a fair estimate.