Any cost to use Apple Watch with cellular funcionality

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

You need an iphone with a watch,without an iphone you can't setup or use the watch with any real functionality.