Information on the Apple watch

The only extra payment you would make is if you have the cellular version of the watch;

What happens is you pay an extra monthly fee on your existing phone contract, the watch uses your existing phone number to make calls in the US it's about $10 per month and about £10 in the UK,that is for the cellular version of the watch, if you buy the GPS only version of the watch you can make calls and send texts as long as you have your phone with you,with the cellular version of the watch you can make calls and send texts without having your phone with you.So in reality your paying $10/£10 a month so that you can leave your phone at home.