It'll only cost what it'll cost to remove the barriers business deliberately erected to prevent their sites being accessible.
The real question is why did business deliberately set out to undermine the basis of the web and to restrict its accessibility when it was actually cheaper and faster to do it right in the first place?
"The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect."
-- Tim Berners-Lee, W3C Director and inventor of the World Wide Web