Today, we’re introducing dynamic sitelinks: automatically generated sitelinks that appear below your ad text, connecting potential customers to relevant pages on your website more easily. This is another example, like selling ratings, of AdWords tools adding value to your ads while saving time and simplifying campaign management. However, it’s important to continue adding and optimizing sitelinks because impression share for dynamic sitelinks will be low. In fact, the sitelinks you set up will always show, except for the few instances when the dynamic sitelink might perform better.Google AdWords Rolls Out Dynamic Sitelinks [adwords.blogspot.com]

Clicks on dynamic sitelinks are free -- you’ll still be charged for clicks on the headline of your ad and other ad extensions. And while they typically boost the average performance of an ad, advertisers always have the option to disable.