Hello...
I would tend to place the value at the high end of what has been mentioned to you ($300-1300), depending of course on condition. Condition is just about everything as far as collectible guns go.
A k-marked serial number would indicate police usage, making the pistol even more desirable. I would suggest checking the websites of places like Collectors Firearms or Simpson's to get an idea of what the sale price would be on comparable pistols.
Cheers,
Gary