Advertisers pay Google to place ads on your site, Google gives you about 1/2 of the payment, most but not all of the ads are based on competitive bids for specific search keyword phrases. some searches like "lose weight" have more "commercial intent" while others like "crayon sculptures" would not be profitable for advertisers and would cost quite a bit less per click.
Based on the cost estimates in the Google keyword tool which is more specifically for sponsored search advertisers, some keywords, like profitable doctor and lawyer terms can charge advertisers more than $50 for a single click.
Premium keyword ads are not sent to regular web sites, it appears that there is a cap to the amount paid for ad network ads, which is what Adsense displays, those trying to create pages about mesothelioma, hoping to be paid $25 a click are probably wasting their time.
Adwords ads that best match the content of a page are automatically choosen by Google, you are better off writing about subjects that people spend money on, a camera review article will cause Google to show more profitable ads on that page than an article about mud ball art.
Testing different ad placements and styles can improve the click rate, generally the ad units should be made to look like your regular content, you can block low paying advertiser's target URLs to improve average revenue.