The story beatguy is referring to has been scrutinized by quite a few people for being potentially bogus, as it was published by a fledgling startup. Some are speculating that the story was either made up for publicity, or that their traffic testing methodology was flawed. Nonetheless, it can't hurt to keep an eye on the quality of traffic you're generating.
Like others have said, what you pay per click is a matter what you bid. Play around with the numbers and see how low you can bid and actually have your ad go live. Start by undercutting their suggested bid by 50-60% and work your way up. Try playing with your target demographic and you may lower your price point of entry. Keep a close eye on the campaign while it's active, because $20 can go very quickly.