I started a trial campaign this week that produced some reasonable traffic at 0.03p per click With no other adwords advertisers yet 53,000,000 serps results I figured it was a pretty good deal but wanted to try and reduce the 0.03p to the minimum 0.01p I changed my keyword from xxxxxx to "xxxxxx" thinking it would be more specific and not generate ads that were not really relevant. I checked the campaign today and found that my bid has risen from 0.03p to £2.50 per click!! Guess what, I have changed it back!! But how come it has risen so much by adding "" even though there are NO Adwords advertisers!!
If the keyword isn't relevant to your site (landing page) google penalizes the site by raising the minimum it charges. You can lower the price to anything you want, google just won't show it. Some 20 cent keywords can require a $10 minimum with the new changes. It's googles latest effort to crack down on sites buying cheap words to drive traffic to sites that aren't relevant for the keyword (effort to increase a better user experience. Factors that determine the minimum bid including the landing page, your ad text, content, number of ads on the page, etc. You can make changes to your landing page, but it can take 3-4 weeks before google will give you a new "quality score". You can do a search about adwords new "quality score" for more details.