It costs 0 by 2020. the free version has no subscription and you can use any controller you want or keyboard and mouse. Google confirmed online gaming on Stadia will be free with the free model, something you need to pay 60 bucks a year on consoles. Many were speculating that online gaming would only be accessible with the sub model.
I know there's a free version but I still don't see why people would buy games for the thing if they cost the same as full priced console games, especially if there is even a hint of input lag or other technical issues.
The free version has no 4K versions, it is a junk method, actually the whole idea of stadia is garbage.