As it depends on the brand.
For example, to service a minute repeater will cost less than a time only, depending on the brands...
But, in general terms, expect to pay some big bucks...
And yes, you're absolutely right to raise this point: It is one thing to buy a complicated watch, it is another thing to service ( or repair ) it.
We should ALWAYUS ask how much we will be charged for a service. But we never think about that, while we often ask when it comes to a car.
To me, it is part of the decision when I consider a watch.
Best,
Nicolas