Isn't it my job as a professional to tell my clients what I am doing and how much it will cost them? Isn't it my job to also explain the importance of these line items? I do this task.
Isn't it my client's professional job to then take that information, process it and see how those line items affect their bottom line? If accessibility for the blind on an Aircraft Pilot app website for Pilots who need vision to have that job and use the app is then excluded is this a moral failing on my part or the clients fault? I would say no in both cases, they have literally zero visitors who are blind visiting their website.
Of course, being competitive, a shoddy developer who just bangs two wordpress themes together will always be cheaper than good craftsmanship, but in my experience, the problem is more often targeting the wrong clients anyway.
[0] And, as sad is this is, the actual design is often not the developers responsibility, unless of course, when it is, in which case he is also to blame. ;-)
Accessibility costs time and money, there is no way around this. Not all companies want to pay that money. I don't think the entirety of the blame can be placed on developers.
If the client doesn't give a crap about it, in the end, a competitor will offer the service without these costs added in, and will win the bidding wars. Adhering to the spec by itself does not guarantee accessibility.
It's not a question of "should it cost something?", but "does it cost something?", and it does.