Long term is that with Stadia, they will keep updating their hardware providing you with the best gaming experiences possible.Part of my PC cost perhaps. I use my PC for many things, I'd have one even if I didn't use it for gaming. I don't see anyone counting the cost of a 4k TV or their Pixel phone for stadia use on top of their Pro subs.
Besides, Stadia isn't going to replace my PC any time soon. There's bound to be games I want to play that aren't going to be available on Stadia (at least on release), especially the more competitive things are going to get.
Even if you want to count my PC costs, I just 'made back' $59 compared to Stadia then. Let's count that as well then. Unless we're only supposed to look at that one moment in time where someone is looking to buy a system and would prefer to save right there and then rather than look at the long term?
4k at 60 FPS is already looking crazy amazing