The cost of accessibility depends upon many factors. If your design starts with accessibility in mind, the costs can be minimal. Keep in mind that the use of

The cost of accessibility depends upon many factors. If your design starts with accessibility in mind, the costs can be minimal. Keep in mind that the use of small and low contrast elements is a design decision. If the decision is made to use large and high contrast elements from the start, there should be no difference in the development costs. Making it possible to scale pages for pages where things are laid out spatially is far more difficult and incurs much more testing, yet costs can be reduced if the decision is made from the outset. Even the more difficult factors, such as captioning images and videos, can be eased if planning is done ahead. The problem with arguing that it's 'mathematics' is that there are many variables involved in the equation. If the variables use values for poor planning and design decisions, the result is going to be a much higher cost. If the variables use values for solid planning and design decisions, the difference is going to be much lower. Of course it also depends upon the media being used, so it is very difficult to use that equation to make generalizations for all cases.