That is, unfortunately, completely untrue. While there are some decisions that can be made (to improve accessibility) that do not impact the cost of development, there are many that do.

While it's true that each small change tends to have only a minor cost, those costs add up when you realize there are thousands of them. They add development time, testing time, and complexity (which impacts maintainability and associated costs).

I think it's worth it in most cases, but it's certainly not free.

The best slides anybody showed were two different building entrances. One of them had a stairwell plus an ugly looking ramp on the side of the building. On the other one they did some grading in the front so that the ground comes up naturally to the door, which looks great and is better for everybody.

Now as I said, this doesn't mean you shouldn't ever bother, but you have to weigh the costs and benefits. Making a product more accessible can get you more customers, which means more money. You can also make the product more appealing to more customers, even non-disabled ones: for instance, suppose you make your software product so that the color scheme can be changed, so that color-blind people can tweak it to see it better. Non-color-blind people may find out about this feature and rave about it because they can also use it to tweak it for their tastes (perhaps with a "dark" theme), rather than being stuck with whatever boring color scheme your designers chose to appeal to the masses. This could drive up your sales, even though the original motivation was disabled people.