One of the most critical steps in increasing the cost-per-click for your AdSense ads is to target high-paying keywords. Google AdSense works by advertisers bidding on keywords on Google AdWords programs, where not all keywords get the same bid.
The popular keywords get high bids, and the less popular keywords get low bids. Having high-paying keywords in your website's content would mean having high-paying ads served via the AdSense program.
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 keyword 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. A free trial is available.
Related: Market Samurai
This online tool gives keyword suggestions and estimated search volume and competition for the particular keyword but doesn't give the CPC for each keyword. It is still a good tool, though, as you can always relate to high competition as high-paying keywords.
Related: Free Keyword Research Tool
WordTracker keywords tool is a paid service for something Google Adwords Keyword Tool gives for free.
Related: Keywords Tool
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