Welcome to the Forum.
First,, to make sure it's an Old Model,, it has to have the old style action. Those were made prior to 1973. Look at it to see if it has 3 screws in the side of the frame,, vs a NM which has 2 pins. Also,, the hammer must have a flat face & not a "stepped" one to accomidate the safety transfer bar. From your description,, it sounds like that's what you have.
That said,,,, to give an accurate price range,, we'd need more info. Condition, condition, condition are key points,, then figure in the completeness of the package such as box, all papers, wax wrap, brown shipper, etc. Next,, if it has any "odd" markings or anything else to make it "different" than other guns. Posting the serial number or part of it can help,,(123XX or 30-123XX.)
More info,, better help!