What is Google earning with every click? What is Google's share?
An interesting question that can not be answered.
Some people suggest that it's around 25-30% but you can't verify it.
It's very likely that it is not static and depends on a lot of factors. Google could for example lower their share in case an ad performs really bad to make the advertiser pay less. That way it would be ensured that the advertiser still has a nice ROI.