How much would it be to add my watch to apple care with my phone

Okay, see this. Buy an AppleCare plan - Apple Support It does not give a cost at the beginning, you probably need to get into checking your device serial number, then your location.

How old is your Watch? You have a limited amount of time to purchase AppleCare.

Also note: If you have AppleCare+ Theft and Loss coverage on your iPhone, and you purchase an AppleCare+ plan for your watch, it does not cover theft and loss. It just covers accidental damage. Still recommended and I surely have it on my watch. But wanted to add that clarification.