The Google Adsense Blog recently placed information regarding how much revenue it shares with the publishers for its contextual ads and search. Google Adsense shares 68 percent of revenue with the publishers for contextual ads while it shares 51 percent of revenue with publisher for revenue that is made through searches.
Adsense claims that Google takes a large portion of the earnings because it has to spend on research, and other costs involved in serving ads to the users. According to the article, the revenue sharing has never been changed since its introduction in 2003.
I think that Google keeps a big share of revenue to itself and even then there is no competing ad platform which can offer almost equivalent earnings to Google Adsense. I expect adsense to increase the publisher revenue since the initial costs to Google were higher with adsense but with an increasing publisher base, it should be expected that their revenue has increased a lot and the benefits should be passed on to the publisher.
Source : http://rudefox.com/adsense-reveals-revenue-sharing-details/