They didn't say anything concrete about money.

I'm guessing developers will be compensated with "exposure" :-)

The "Stadia store" suggested games would likely be sold or subscribed to. But I definitely have to imagine some sort of trial model being available for the launch-from-YouTube options. Perhaps a time-limited option, that won't stop you from joining a streamer with Crowd Play, but would cut you off after a while if you hadn't already purchased it?

I'm very curious about the expenses game developers will face too, which definitely wasn't brought up. Id Software bragged about just using one Stadia instance, but then later demonstrations showing split screen gaming "without compromise" was shown running each screen on a different Stadia instance. While Google suggests this makes split screen gaming possible again, my guess is that split screen gaming costs the developers twice as much to implement as a service cost, since it uses twice as much hardware. Making it probably mostly a nonstarter anyways.