Totally depends how you're defining "accessibility", as it's a very grey area.
For instance, to some companies accessibility means a website working in IE8, Chrome and Firefox. To others it means working as intended without JavaScript or images. To other it even means the fact that they've passed the online HTML validator (big lol btw)
To others it goes a stage further and accessibility implies WAI AAA compliance - have a read of the
WAI guidlines
It also depends on your client. I do quite a lot of work for local councils, where maximum WAI compliance forms a good chunk of the brief. So doing things like testing the site using greyscale, screenreads etc. is a must. If all priority 1,2 and 3 WAI points aren't met I basically not fullfilled the brief. However, for a local plumber's 3 page website, WAI AAA wouldn't come into it
In my opinion, it's one of those area's that are very similar to SEO - very grey, hard to define and constantly gets abused