321 Service dials are in my opinion, are interesting and collectable. So far, they remain without value.
The heart of the value is the dial. It must be original, and then it must be attractive. If it is a service dial, and attractive, it might add a couple of hundred dollars to the base, (without dial) value. An attractive 145.012 dial might command 1500 - 3000 on its own. Could be more if it is perfect and exceptionally attractive.
As the heart of the value in these watches is in the appearance of the dial, then really the remaining parts of the watch are easier to value as the sum of the parts.
Service dials, like all dials, can be attractive or ugly. For me if a service dial is aged nicely, it is as attractive as an original, perhaps more attractive than many - however it will never command the dollar$ of a collector who seeks original.
Ironically I think service dials are rarer than Original.
Some calibre 321 Service Dials. The one on the far right is exceptionally nice. Still almost valueless! :