Apple Watch

If you buy a GPS+ Cellular Apple Watch and your cellular provider supports Apple Watch, you can buy a data plan for the watch, which does include a monthly charge.

If you buy a GPS only model, you cannot add a cellular data plan, so there would be no monthly charge. But your watch will require your iPhone be nearby at all times to work.

Yes,in the US it's about $10 a month and this is added on to your monthly phone contract.

Make sure your network provider supports the cellular apple watch before you buy anything.

Here is a list of providers that support cellular watch;

Apple Watch - Carriers

You're very welcome.