Does it increase income? All things being equal, a site that is better ranked in search engines for their target keywords make more money if it can convert that traffic into money. This is the big catch. Converting visitors into money isn’t as simple as getting new visitors. Getting traffic to convert into sales/ donations is a lot more involved than merely getting people to the site.
Does it save money? Better inbound referrals that arrive from organic means will certainly save money if you’re spending money on CPC advertising. Organic traffic is "free". Traffic that arrives from online ads and pay-per-click ads are definitely not free.
Does it mitigate risk? Not really. If we consider "competition" as one of our risks, then I suppose being ranked better than your competitors is some risk mitigation, especially if you’re in a stiffly competitive market.
How strong is the evidence? Medium. The strength of accessibility’s SEO benefits are highly dependent on how valuable a single conversion is to you.
I happen to think that the SEO benefits of accessible development are overstated – especially because the only possible SEO benefit from accessibility is in the form of on-page factors. Based on my own analysis of WCAG 2.0 Success Criterion, only about 27% of the Success Criterion have any relationship to SEO. On the upside, 73% of the SEO/ Accessibility best practices are Level A and 8% of them are Level AA. If anything, I’d argue that improving on-page factors for SEO improves accessibility, not the other way around. Let’s face it: Things like better focus control, better form labels, better error handling are not going to increase your SEO. These are vitally important to users with disabilities, but offer no benefit for SEO.
That being said, I’m very intrigued by this topic. I once had an off-the-record conversation with the SEO & Accessibility Guy at a Fortune 500 company who regaled me with tales of how much one quality conversion is worth to his company. I am currently finding a way to gather hard data on SEO and accessibility, because it seems promising in certain situations. To get an idea of the ROI you can get from SEO, use this SEO ROI Calculator. One thing is certain: you cannot offset poor SEO with PPC cost effectively. Improvements to SEO can mean huge money, depending on the type of company and type of website. Apart from the effort to design and maintain a search-engine friendly website (which would be the same as any other site) or to improve a current site’s SEO, organic traffic is essentially free. Conversely, PPC and other advertising always costs money and is less likely to deliver as effective ROI as organic SEO traffic. You may be able to kill two birds with one stone by increasing SEO and accessibility at the same time.
What would make this evidence truly compelling is research which can directly connect improvements to the site based on stated SEO-to-WCAG mappings and track increases in organic traffic and conversions. The thing that most often makes this a weak business case is when the site is unable to convert traffic to business. New search engine referrals don’t do you a bit of good if the new traffic doesn’t mean new money. Sort of like the old "If a tree falls in the forest…" if the visitors don’t convert, is it a business case? My answer is no.
For VIDEO, You can go to http://sites.uci.edu/sbass/professional/benefits-of-accessibility-and-seo/#comment-9388