If you buy an Apple Watch with a cellular radio, then you pay more for that specific model, because it has extra cellular radio hardware and the built-in eSIM inside. You may also be paying more for a different more expensive case, or a different more expensive watch band, or because you got the Nike Branded model. This is a one-time upfront cost.
YOU DO NOT pay anything extra to a cellular carrier if you have an un-activated cellular radio. You ONLY pay a cellular carrier if you activate the cellular radio, and that is generally a monthly fee as long as you keep the cellular radio activated (In the U.S that is around $10 per month).
If you buy a GPS only Apple Watch, then you do not pay extra for an unused cellular radio. But you cannot add cellular to that specific Apple Watch in the future either.
I have been using a GPS only Series 4 for the past 3 years, and I have been happy with it, as my iPhone is almost always with me, so I get all the functionality, with out the extra cost.
My Wife has a Cellular Apple Watch, because she does not want to carry her iPhone with her all the time.