According to the results of the iProspect Outsourced SEO Metrics & ROI Study (PDF link), 35% of the surveyed organizations that promote their web sites with natural search engine optimization (SEO) and pay per click advertising recognize a higher return from SEO.
This compares to just 11% of marketers who report that PPC ads produce higher ROI. That means that three times as many webmasters who can measure the ROI of each method recognize a higher ROI from natural SEO than from PPC advertising.