One of the basics of SEO is that it is a long-term strategy that delivers a return on investment (ROI) long into the future. Whereas most paid advertising campaigns only deliver ROI dependant upon your ongoing investment.
The graph below illustrates the power of SEO and the compounding impact it can have on your website/business.
The Return on Investment (ROI) of SEO vs PPC. Blake Smith, blakesmithy.com