That is about the situation, and the same is true for the numerous "limited edition" police guns put out by S&W over the years. These were usually made up in limited numbers (200-500) for sale to police officers to support police benevolent associations and the like. In the area (state/county/city) engraved on the guns, they sometimes bring a slight premium; outside that area, the marking actually detracts from the value.
Even the "official" commemoratives (such as the Model 19 Texas Ranger) don't bring any huge amount more than another gun of the same model and same condition. For example, the TR, new in the case, will sell for around $600, while the same gun without the TR marking and in the plain box will bring around $400.
Jim