CPC is the amount of money which you have to spend for a click on an ad when initiating PPC or Pay-Per-Click campaign activity. When you will take part in the bidding process of Google to buy AdWords or keywords, you will find that though each keyword does have an estimated click cost, but, the prices change when the users bid against each other to buy a keyword. Average CPC of an unpopular keyword is very much low compared to the CPC of a competitive keyword.