It's all based on how many visitors you get vs. how many of them click on ads. It also depends on how much each advertiser pays for clicks.

For instance you could have a day where only 5 people visit and only 1 clicks on an ad and you make $1.00. You could also have a day where you have 5,000 visitors and 150 of them click on an ad but you only make $0.10.

The key is volume though. You have to get a ton of visitors and a lot of clicks to make any money.

You might be better off going with Text-Link-Ads. They pay a flat monthly fee for text-link ads placed on your site, regardless of how many clicks you get. The key (again) though is you have to have a higher-rated/volume site in order for anyone to want to place an ad on your site to begin with. They use Alexa and Google rankings to figure out a "value" for your site.
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The Graphic Mac - Tips, reviews & more on all things OSX & graphic design.