Hard to believe that google would do this for free

By offering 1080p with stereo sound for free those people that doubt the service will be able to try it out for free. For 5.1 sound, 4k, among other perks, the users will have to pay 10$ per month.

If someone can offer 1080p for free it is Google. There are other smaller companies that have been offering a similar service but they are very expensive. With Stadia most people will surely be able to afford it.

maybe they have plans to charge more for it later

That would really suck, I think competition will keep the prices very affordable.

fill most of the screen with adverts that you have to pay to make go away

This will guarantee 100% the failure of this project, there is nothing most disturbing for gaming that suddenly having an ad interrupting the experience. I don't think that will happen.

Also, I don't recall seeing ads on Ready Player One?