Hi, I tried to use the keyword "yahoo answers" for a website where I sell yahoo answers e-book, the first time I use it the price was 0.05$ and between night it became 5$ , how come ?? I am the only one who used that keyword. it so unfair ...
You've been slapped. Welcome to the cruel world of PPC. From what I hear, you most likely are advertising on Google. Spread your eggs out to MSN and Yahoo. They're slightly more tolerant on the QS issue. And buddy, life is never fair.
...and Google's QS system seems so grossly unfair and un-thought out, it's a wonder anyone uses AdWords anymore. How many cases have there been of someone that has a genuinely good, helpful, popular landing page/site, but Google's "secret spooky algorithm" decides otherwise, and disallows that person from advertising? I know some people don't have the quality sites they think they do, but that doesn't change the fact that many do and the system is, as prosumer86 said, cruel and unfair. But there is competition for AdWords!! If they don't give good return for money, they will eventually disappear; it will take care of itself thanks to the market. Bigness -> hubris -> out of touch -> no more AdWords.