A few things you must understand with values shown in the keyword tool.
1. It's meant for Adwords advertisers on the search network. That is, advertising on the Google search engine itself. As an Adsense user, you are a publisher on what they call the content (or display) network. This is very different than the search network. Being different means that CPCs are different.
2. The GKT is an estimate of how much an advertiser would have to BID to be in the top three positions on the search network. What they actually pay is based on their Quality Score (QS). The higher your QS, the less the advertisers generally pay. The keyword tool simply uses an average QS to calculate the estimate.
3. It's a very dynamic system. Because of that, CPCs can be very dynamic too. Dozens of things can affect the CPC at any one moment. The same dynamics exist for the content network.
4. You think Google gets $1 from an advertiser and gives it all to you? The stated figure I believe is 68%. Pretty good, you get most of it.
5. Because Google wants the best ads (higher QS which is mostly based on CTR) and rewards them with lower costs, you want the same. It's easy enough to calculate, they make more money that way which means you make more money. Let the system serve you the ads it thinks is best and you'll maximize your revenues, all else being equal.
