What the work actually looks like
In the stores we work on, accessibility failures are rarely spread evenly across thousands of pages. They cluster in a handful of shared components: the header and mega-menu, the layered navigation filters, the product options block, the cart drawer, and the checkout form. Fix those five and you have fixed most of the catalogue at once, because every product page renders the same code.
The second cluster is third-party scripts. Review widgets, live chat, cookie banners, and payment iframes all inject markup you did not write, and they are a common source of unlabelled buttons and keyboard traps. They also update on their own schedule, which is why a store that passed an audit in March can fail in June without anyone touching the theme.
That is the real shape of the cost. The first pass is a project, usually a few weeks of front-end work on a Magento or Shopify storefront rather than a rebuild. Keeping it is ongoing maintenance, the same as security patching. Treat it as a one-time audit and you will buy the audit again next year.
It is also the same engineering discipline that shows up in conversion rate work. Unlabelled fields, invisible error states and controls that cannot be reached by keyboard hurt every shopper on a bad connection or a small screen, not only the ones using assistive technology.