Nick, does it look like this;
Or does it look like an actual carbine.
Or does it just have a long barrel? Many lugers could be ordered with "long" barrels during the 1920's and early 1930's and they are just long barreled lugers. (please note the spelling of the gun)
Value depends on what it is, a long barrel luger, $1200-$2000
Pictures are the only way to really 'guess" at a value without seeing it in person.
ed