You ONLY get the iPhone plans offered by the cellular carriers that support the Apple Watch. You do not get to choose a pay as you go plan, unless the iPhone carrier supports the Apple Watch on that plan.
The Cellular + GPS Apple Watch shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:
https://www.apple.com/watch/cellular
T-Mobile has an age 55+ plan for 2 that you can add Apple Watch support. If you are willing to have limited Apple Watch data usage, you can get the T-Mobile Apple Watch add-on plan down to $5/month on top of whatever T-Mobile plan you have. Downside is that T-Mobile coverage, while good, is not as deep as Verizon or AT&T. That is to say, if you are in some rural areas you may have a weak T-Mobile signal. But if you are in range of your home WiFi, T-Mobile allows WiFi calling so you are good while at home, but out for a walk may have spotty cellular coverage (my Wife and I have this setup, and while we live maybe a mile and a half to 2 miles perpendicular to a major 6 lane highway with great coverage, our neighborhood has spotty coverage).
Comcast/Xfinity supports the iPhone and Apple Watch. I'm told that when added to your current cable plan, it is not too expensive. A guy at work has been using it. NOTE: Xfinity uses Verizon as its cellular carrier.
NOTE: If you like to visit Maine, then Maine residents report Verizon has the best coverage. My experience with T-Mobile is that it is not terrible, but it can be spotty, and sometimes you are on a partner cellular service with voice only, and no data when visiting some less populated areas in Maine. The obvious tourist and major population areas are well covered by T-Mobile.
See the above list of carriers that support the Apple Watch.