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.