SEO > Adwords
Outsiders often estimate the costs of search engine optimization to be quite high. Yet it is significantly cheaper than generating targeted visits via comparable pay-per-click offers such as Google's "Adwords" ad program or the "Yahoo | Bing Network" program.
Moreover, the discounted cash flows that can be generated by effective, long-term search engine optimization are hard to beat.
Clever Optimize
It is often possible to acquire particularly valuable visitors (e.g. buyers for an online store) at low cost through clever search engine optimization.
Thus, instead of general keywords (e.g. "shoes"), one could focus on very specific keyword types (e.g. "low shoes") and combine them with words that signal a willingness to buy (e.g. "low shoes store"). Thus, higher returns can be achieved through search engine optimization by clever keyword selection.
Optimizing for three such specific keywords, for example, could then cost as much as optimizing for a single more general keyword, but attract much better (e.g., more willing to buy) visitors.