It all depends on condition. 2 19-4's shipped at the same time, one could be worth half as much as the other due to how it was used and treated by previous owners. A NIB one may be worth $700, and a well worn, heavily used one may be worth $300.
With nickel finishes, the conditon of the finish is very important. Does the finish have milky or cloudy areas? Are there any pits in the finish that could lead to flaking? Does the M19-4 have the N stamp on the frame? Do the grips/stocks serial number to the gun? Lots of questions I know. But it is hard to determine value without more information.