$15/m for an actual Google+ service where I'm guaranteed that I'm not being tracked, and I'd get no ads and all my personal data is completely safe and encrypted doesn't sound that bad.

You've rather blatantly cherry-picked those figures, why would you use the figure for 300m in-stream Google+ users, rather than the 540m figure? Even that is an underestimate, because Google serves ads to many more people than directly use its services.

Yes, but Google could still collect ad revenue on non-tracked users, they would just be restricted to the same kind of ads they use to target non-logged-in browsers. Obviously the ads within Gmail would be worthless on an untracked user - nix those. But the ads on the Google search pages? On sites that host AdSense (or whatever they call it now)? They would still have tremendous value.

I do wonder if this wouldn't break Google Now though.