Servicing costs will be dependent on where you have it serviced, how complicated the movement is, and whether or not any parts need to be replaced. In most places, a routine service on something like a basic ETA 2824-2 will run $200-$300.Just curious; What does a normal watch service consist of and what is the cost range?
I was reading that watches with the Seiko NH35, 36 or 38 movements or even the more expensive Miyota 9015 movement it is probably a better route to just have the movement replaced if and when it starts to show issues rather than actually having them services at regular intervals.
You could also buy a new movement yourself, for roughly the same cost, and drop it in yourself, with a little know-how and the right tools.
The Seiko NH3x's can be purchased new online for about $40-$50. A new Miyota 9015 might be around $100-$120. A watchmaker might charge $50 to drop a new movement into a case for you (or, again, you could pretty easily do it yourself, with the right tools). As such, those movements make less sense to service, and more sense to simply replace, once they die.