There is an all-in-one article including every part from this series (if you want prefer to read it all at once instead of "by installments")
Is it expensive in time? Expensive in money? Both of them? Either way, it is a dubious claim that can be heard towards the end of projects in which they did not consider Web Accessibility at the early stages... which makes it expensive! Teams will have to work on issues after the fact, rebuilding the solution (almost from scratch in some cases), which will waste time and money. Something that they could have avoided if they had implemented accessibility at the start.
If you have a car and the oil light goes on, you change the oil. It will take you a few minutes, either done by yourself or at the shop, and it will cost you just a few bucks. However, if you don't change the oil, chances are that your engine will seize up and break down after a while. The repair will be in the hundreds (or thousands) of dollars, and it will require a mechanic using special tools and parts. Not to mention that the car will be off-service for days or weeks.
If you apply Web Accessibility at the right time (at the start of a project), it will cost less in the long run and save many headaches and stress to the team. But if you wait to do it until you have to (e.g., after users complain or you get a lawsuit), then it will be expensive and painful. In addition, it will require specialists and be time-consuming.
Taking into consideration that people with disabilities have a purchasing power of 1.3 trillion dollars (over 8 trillion including relatives and friends), maybe it's time to stop talking about Web accessibility as an expensive feature, and start presenting it as a profitable one.