Google does not disclose exactly how much you'll earn per ad that is clicked.
The commission you receive per click depends on how much advertisers are paying Google for the particular ad. You will earn a share of that amount. I've heard of earnings anywhere from 2 cents to $15 per click.
So it is logical to believe that keyword phrases like debt free, employment, make money, mp3, sex, etc. will earn you more per click since these are highly competitive keywords that are searched for quite a bit on the web.
Advertisers generally pay more for popular terms because they are searched for more.
Even though Google will not reveal how much you are earning for each ad that is clicked from your site, you can still login to your account at any time and see the total amount of revenue you've generated that day, week, month, year, etc.
For example, if you see that you've made $12.60 today from 9 clicks then you can calculate that your average click-thru commission was $1.40 per click. That's as detailed as their stats will get. Also remember, that's only an average. You won't know how much each specific ad brought in.
The amount you'll earn also depends largely on the amount of targeted traffic you receive to your own site, how well the ads match your audience's interests, the placement of the ads on your pages, and of course the amount you receive per click.
Ideally, you should create a site on a topic you know a lot about. That way you'll have a much easier time creating a generous amount of content on that subject.