Have yet to test this new analysis tool from SpuFu.com, but it looks very promising. A mentor of mine Paul Allen, said this method is one of the best ways to validate the use of SEO inside of a business. Using the dollar spent data from PPC, one can determine the value of ranking organically for that term.
Some studies I have read, even say that you get a even more traffic for having both PPC and organic listings for the same search term. I know for a fact that having multiple web pages listed organically in a SERP, such as a double or indented listing can bring double the amount of traffic. So adding PPC or adding getting ranked for a search term you are bidding on can actually have an almost exponential effect.
This is probably more true with less web savvy users of search engines. As they pay less attention to the advertisements on websites. So you might see more success with this tactic with people searching for “cute yorkie puppies” vs those looking for “xml sitemap generators“.
I merely dabble in PPC so I have yet to test this for myself. But many of my comrades in arms claim it is a valid strategy and wroth testing. Choosing those words should be easy using this tool and your own internal data.