Lets start at the top.
Open the cylinder and look inside the hinge bar (Crane) and see if there's a model number there.
It should look similar to this Mod 340-2 or something similar.
Second, the Serial number is on the bottom of the butt grip. we would like tha 1st 5 number and letters in the correct. Use x'x for the last set of numbers in the serial.
The barrel is measured from the very front to the very back where it meets the cylinder inside the frame.
Photo's will be of a great assistance.
The value can vary greatly based on where uyou're located. Pricing can swing as much as $110 or so, depending on regional supply and demand.
We'll get you all the help you needs with a little help from your end. once we have those basics we can get you on track pretty quickly.