I believe the amount you get paid per click depends on the link that gets clicked. Since I think advertisers pay to lock in keywords, usually the bigger companies will pay more per click, whereas the obscure links will pay less.
I can't verify this 100%, but as an advertiser and working with an ad/banner company, that is how it worked. You would bid on keywords by how much you were willing to pay when someone clicked on your link.
So for example, I may have bid .50 on the word "webhost" and .75 cents on the word "mysql", etc. Then, the higher you are in the bidding, the more often your banner would be shown.
So it was my experience, that the companies that could afford to do so, would bid highly on the important keywords to ensure their banners were displayed more frequently than others, but at a higher cost per click.
Hope that helps!
