I'm not quite sure that I understand the question. As far as I know, ADA compliance is related to hospitals and hotels who has certain accessibility issues. I don't know if there are certain rules for websites in US. But probably my lack of knowledge is due to not being American and not living there.
I assume that you want to build accessible websites that do comply with the regulations that a govermental site should follow.
The way to price for this is exactly the same as with anything else. You're providing added value, an extra that may increase the traffic of the site (and that means more revenue and more possible customers).