Presume you currently with EE then Not too much but think that is what they call the sort of combined phone / watch type plan where the watch then shares the same number as the phone and is included in the phone's usage of minutes, data etc - is a few pounds extra a month over your phone plan ( excluding cost of watch )
As far as I know ( but again would check with your carrier ) you can either buy the watch through the carrier or I believe you could maybe buy the watch independently from Samsung, Argos, Currys etc and have it included with a one number plan with EE but as I say would confirm with EE first
I can't remember what carrier but did hear of a chap a while ago who bought a watch and only discovered afterwards he was with a phone network on which he couldn't add his watch - hence my comments of checking with your carrier first just to be sure !