Google never used to share these number no? I remember people always speculating what they were doing
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. We don’t disclose the revenue share for other AdSense products; the revenue share varies for other products due to different costs of developing and supporting these products.
We believe our revenue share is extremely competitive. However, revenue shares alone can be misleading, so we encourage you to focus on the total revenue generated for your site. For example, if the Google ads on your site generate $100, with our 68% revenue share you’d receive $68 through AdSense. Another ad network might offer an 80% revenue share, but only collect $50 from advertisers, so you’d receive $40.
With the vast number of advertisers competing to appear on AdSense sites, our system ensures that you’re earning the most possible for every ad impression you receive.
Its been disclosed for awhile now (atleast a couple years), and even longer in general terms in their quarterly financials.
Also – they take 49% of Adsense for Mobile.