blomman Mr Blue
38586
Welcome here, Tony!
And congratulation to a very fine and rare piece!
As for the service cost, hard to tell since it all depends on what has to be done on your watch.
The good thing with sending the watch to GP instead of a local watchmaker is that the chance of them having the correct spare parts are much higher than at you local watchmaker + they are experts on the movement!
If you could post some photos of your watch, maybe we could contact the Manufacture and ask for a rough estimation?
Best
Blomman