The best way to justify investment in organic SEO is the fact your customers don't trust ads
The infographic from seotraininglondon shows just how important ranking well organically is for your business. Consumers are far more likely to trust links that aren't ads. People think organic search results give answers that are more relevant, and are thus more likely to click on them. Only 28% of people think organic results are more reputable than Ads, so in terms of reputation PPC may not be such a bad thing after all.
Check out the infographic based on 1,000 respondents to an SEO survey. It's perfect if you need to justify increased spend on organic SEO to a manager.