$10/mo was pretty much the only way they could price the service; however I didn't expect 4k to be the base resolution for the paid service. (thought it would be $10/1080p, $15/4k; having 1080p for free is interesting).
Google needs to be more transparent about device support though. The stream highlighted Chromecast Ultra/Android, but not much else (e.g. if there is an iOS client, or compatibility with a base $35 1080p Chromecast/Chromecast protocol for smart TVs) [EDIT: the FAQ is more clear: https://support.google.com/stadia/answer/9338946]