Pay-Per-Click Ads are exactly that. You pay if someone clicks. This method can sometimes seem expensive at first, but if you have a well written ad and a good looking site, the returns can be high.
Figure you are paying around $5 per click and you get 100 clicks. Yes, that is $500 bucks out of your pocket for advertising. YIKeS!
You have to look at it this way. What is each paying customer worth to you? Say you are a roofer and a simple repair job is worth about $300 and you get a 2% of your clickers to buy from you. A very low %, but these two customers just spent $600 on repairs which is the lowest paying service you have. You just made $100 for the month... Imagine if you had more than that buy from you. You would be in the green.