One of the most important step in increasing the cost-per-click for your AdSense ads is to target for the high paying keywords. This is because Google AdSense works by advertisers bidding on keywords on Google AdWords programs, where not all keywords get bid the same.
To put it simply, the popular keywords get high bids, and the less popular keywords gets low bid. To have the high paying keywords in your website's content would then mean having high paying ads served via the AdSense program.
There are few ways to search for high paying keywords for Google AdSense. Some of the are listed below.
There's nothing better than a tool from Google itself. Google Adwords Keyword Tool is initially a tool for Google AdWords advertisers to get insights on keywords targeting by displaying relevant information about searched keywords such as the search volume, Google AdWords advertiser competition, and of course, the CPC.
Market Samurai provides the same solution as the Google Adwords Keyword Tool, except that it's a Windows program and is not free. Free trial is available.
This online tool give keyword suggestions along with estimated search volume and competition for the particular keyword, but doesn't give the CPC for each keywords. It is still a good tool though as you can always relate to high competition as high paying keywords.
WordTracker keywords tool is a paid service for something Google Adwords Keyword Tool give for free.