I looked at T-Mobile's site after the announcement, and they list $10/month for "wearables". Until more details come out, I'm going to assume that's directed at the Apple Watch (can't think of any other wearables that have a SIM. EDIT: apparently Samsung: https://news.ycombinator.com/item?id=15229591). No word on what a standalone watch would cost, but I think it a safe assumption that it would be what a single mobile phone line would cost.