Yup, totally free (with a Google account). It's run as a learning resource and there's an upsell to Collab+ and Collab Pro, but for running StableDiffusion it makes it very easy to get started!
I think it's less of an upsell and more of a data collection and a market positioning. Google would like to, for example:
1. Be central in the machine learning ecosystem. This has broad ripple effects, such as recruiting.
2. Doing things there means Google can track how you use machine learning. This can be used for everything from understanding trends in machine learning, to, again, robustly identifying individuals for recruiting efforts.
It seems like the cost is nominal at Google scale for what Google is getting. I suspect the pricing for the higher-end services is less a money-making scheme, as at some point, free is no longer sustainable (and if unlimited CPU were free, that would be prone to abuse / misuse / overuse / wasteful use). The amount of money Google makes there is nominal at Google scale.
