Having read some books & articles on behavioural economics, some of the economic terms have inadvertently permeated my brain and I’ve been thinking about the accessibility applications.
Return on investment (ROI) for accessibility is difficult to assess at both ends of the equation, the return and the investment. But there is another cost that people tend to miss.
Some of the costs of accessibility can be seen (in economic terms) as 'externalities'. In the domain of energy production the cost of producing petrol is the obvious one, but the cost of pollution to the population overall includes health issues and climate change. As the original supplier doesn't pay for health treatments or the impact of climate change these are 'externalities' (a cost that doesn’t affect the people who create the product).
In accessibility, the externalities are forced onto the end users. If you create barriers in your website for people with a disability the cost is borne by them, not the website owner.
However, the website owner can see both the return and the investment affected in reduced revenue (people who can't access the content) and the unlikely event of a legal claim.
I say that a legal claim is unlikely, however, I do know that large organisations can be put under pressure to improve before a legal case is made, so the cost can be that you have to retro-fit accessibility on someone else's timescale – the expensive method!
When you consider that accessibility is regulated (i.e. it is illegal to provide a service that is not available to people with disabilities), everyone should be on a level playing field of making accessible websites by default.
Therefore an organisation should bring those externalities in - build accessibility into the process and make it the default approach.
When we train teams in accessibility that programme should be the only noticeable cost, the only accessibility line-item. After that, the design and development teams should build accessibility into the scope and timescales, it is not an extra cost.
There might be a small increase in timescales if you could compare the same feature before and afterwards, however, what you have done is an honest appraisal of the cost including the externalities.