The most easy way is to compare organic traffic before and after SEO as well as average SERP positions by providing keywords. By Analytics and provided by SEO expenditures we may calculate Cost-per-Lead or Cost-per-Action for people, who came from search systems. However, you need smart analyses conducted, taking into account other channels (PPC, Social Media) and full consumer path among them. We will help with that, because, science - matters :)