I have heard of cases where increasing contrast to meet accessibility standards resulted in significant boosts to sales. Proper contrast is beneficial to people with vision impairments, but it also seems to help people without vision impairments navigate a UI. I would guess that you could also see a big sales boost when you make your site fully accessible to the colorblind - something like 10% of males are colorblind.

When it comes to screen readers, the percentage of customers who use them is much smaller, but there are lots of business reasons to have a good experience for blind customers: it's important in markets like education and government, and it avoids bad PR.