For your background: AdSense has two products: one is AdSense for Content, which allows publishers to generate revenue from ads placed alongside their content.
The other is AdSense for Search, which allows publishers to place a custom Google search engine on their site and generate revenue from ads shown next to search results.
The majority of AdSense publishers are using the content product, says Google. Publishers apparently earn a 68% revenue share worldwide, meaning Google pays 68% of the revenue that they collect from advertisers.
Since launching AdSense for Content in 2003, this revenue share has never changed. For Search, partners see a 51% revenue share for the search ads that appear through their site. The AdSense for search revenue share has remained the same since 2005, when Google increased it.
While Google says it has no plans to change the rev share for now, it can’t guarantee that the revenue share will never change in the future.