How much Google share with adsense publishers used to be a blackbox but not so anymore.
For displaying ads with AdSense for content, publishers receive 68% of the amount Google collects from advertisers. For AdSense for search, publishers receive 51% of the amount collected from advertisers. These percentages are consistent, regardless of a publisher’s geographic location, and are not in any way averaged between publishers.
If you have an adsense account, you can also see that under My Account > Account Settings