Yes, I agree with Jigartank! If you have a 2% CTR (middle-high value), if you get 1000 visits and your CPC is 1$ you can earn 20$!
The problem is that to get 1000 visits you need 1000 clicks from US people (if you're there) and this costs from 0,50$ to 4$ (depends on targeting)!
So your campain will give you 20$ with a 500$ investment. So there is no way to earn if you plan to make money with AdSense!
But i have a friend that run facebook advert. He earns more than $500 daily. He refused to disclose the trick he uses.