If you were to take the sum of all CTR it would average out to about 2.5%. Though this number in and of its self really doesn't mean a lot. Some industries have double digit CTR while others 1% is golden. Just depends on the niche and the competition. I do agree with Lucid that CTR doesn't mean anything unless you can match it up against conversions and in the end profits.
Based on my experience, CTR is directly proportional to Bounce Rate. As your bounce rate goes lower, there is a high chance that you will get higher CTR. That's how i analyzed my adsense and analytics data. How about you?