Stadia Base as mpg1 mentions is free, no? Play on any device, no contract, just buy a Stadia game and play on anything. That's a new business model. Realistically, it's not tenable outside of someone like Google using that Cloud Compute for other stuff as well. $60 for a game and no running fees, on hardware that Google has to buy and operate, has to be a loss-leader for Google propped up by something else. Either that Stadia hardware is used for other stuff too and just used for gaming as a sideline, or Google have some longer term ambition that'll have far higher earnings potential, such as one day when 100 million are playing Stadia Base, introducing a $5 a month fee.
Also an obvious but key point with cloud is user base scaleability isn't tied 1-to-1 with hardware units. You really only need to match peak active demand at any given time. Google wouldn't need a 100M+ units of hardware to achieve the same "user base" as PS4.
So a single Stadia user is not contributing to the total cost of a single hardware unit. On average it's probably 3-4.
If Google uses Stadia instances for other compute tasks like youtube costs per user is driven down again.