The increasingly popular Google search engine introduced a cost per click (CPC) program called AdWords that is just a cut above the rest. This is not the usual "highest bidder wins scenario". They differentiated their service by adding a quality element. It means that those ads that receive a higher click through rate get a slight reduction in price. Some of my own ads produced a good reduction in cost so persuasive writing does make a difference.
Google's CPC program is complicated, even troublesome at times. However, it does have an automated feature called AdWords Discounter, that allows you to let them manage the bid price at a competitive level. Some of the more general and well used keyword phrases have a set minimum bid which makes some phrases very unattractive. That's why you won't see ads on Google for certain keyword phrases. It's just too expensive given the type of sales transaction that results from clicks on that phrase. The word free can sometimes be very expensive on Google AdWords Select
To make matters worse, there are popular, general phrases that are less targeted. This means they develop less qualified visitors and thus a poorer sales conversion rate. Google's broad matching system means wide varieties of buyers are competing for the same phrases, even though they are selling completely different products or services.
This often results in bidding wars and demonstrates why you need to use a systematic testing approach is necessary for AdWords to perform well for you. If you don't have a system for discovering the best quality keyword phrases for you, you'll be spending a lot more money. On a large campaign it is essential to quickly discover what makes people click. Another reason is click fraud. There are estimates that 20% of all click-throughs are fraudulent, so expect 20% of your budget disappearing.