If you consider accessibility at the beginning, the cost of making content accessible should be minimal. Accessibility benefits everyone and should never be thought of as an expensive "added extra". If you add accessibility into your design process early. it pays dividends downstream in compliance testing and via inheritance in the code.
No one wants to retro-fit their application. It's difficult and frustrating process.
I drove the sales cloud accessibility compliance effort. Designing test plans, serving as point of contact, working across development teams to educate as well as coach them through the process of meeting the WCAG 2.0 and Section 508 standards. (Markup validation, Screen readers JAWS, VoiceOver, TalkBack)