I could go along with that, if accessibility actually cost more.
It sometimes costs me some testing time, but not much. It doesn't cost extra time coding at all (it might if I were bolting-on, but not if I'm in at the start and doing Built-in).
It sometimes costs me time researching (learning), but since that increases my value I'm not unhappy with that. The more I know, the better sites I can build.
And, this may be a matter of opinion, but I don't see the point in building sites that are broken for some. I don't consider it an extra thing the client has to ask for (accessibility, usability) any more than I pay extra for my car to have brakes. They should just come with brakes. And lights. And a horn. And paint on the outside. And seats inside.
did I just say "leopard striping"? Good thing I'm not a zoologist