Your CTR will depend on a number of different factors

JoelComm mentions 4 factors that they could affect considerably your CTR, he passed from a 1% to a 8%

Content of the site
Some types of content generate more clics than others (but, they do not necessarily make more money by clic).

Design of the site
It is very important to know where and how to put them

Number of links
Why to make competition to your ads? If you want that people make clic outside your page, you would have to be paid for that reason.

Relevance of the announcements
If your ads are not relevant, you have a lot of chance that your CTR will be the lowest, in my opinion, it is the most important, because people do click to the announcements that are related to the subject.