From Elegant themes
Found this article to be both relevant and useful:
If you want to promote your website, online ads are one of the best ways to do it. With AdWords, you can reach one of the world’s largest audiences, as long as you have the budget. The only problem is, budgeting for AdWords costs can be complicated if you don’t know how much you should expect to pay.
Luckily, there is a lot of AdWords data available to the public. Plus, you can set up an AdWords account and test it without spending a dollar. Today, we’re going to talk about how AdWords calculates prices, how much clicks and impressions cost, and how to budget accordingly.Let’s talk numbers! Link to blog posting