AdSense works based on Google AdWords (http://adwords.google.com). Basically, advertisers bid on keywords ... anywhere from 5 cents to a few dollars per click. When the keywords are found on a page containing AdSense, the advertisers' ads are shown. When someone clicks on an AdSense ad on your site, you receive an undisclosed percentage of the bid. Therefore, you receive a different amount for each ad. There is no way to know which ads are high performing and which aren't. In general, certain competitive keywords are worth more than others, so it depends what your site is all about