SEO : Paid vs. Organic Search

More source:

1. http://www.optimum7.com/internet-marketing/paid-search/paid-search-vs-organic-search.html
2. http://blog.hubspot.com/blog/tabid/6307/bid/1514/Paid-Search-vs-Organic-Search.aspx
3. http://www.wilsonweb.com/paid-search/organic-paid.htm
4. http://searchengineland.com/paid-vs-organic-search-understanding-the-dynamics-33155