I don't think that is a good example. If the treatment you need requires you to fly, that means it a very complicated diseases, or a rare disease that only a few centers treat. In any case, it is likely that the care is very resource intensive and is either expensive or heavily subsidized. In those cases, even if the airfare were doubled or tripled, it would not be the limiting factor in treatment.
Example: I live in the Philippines. Its healthcare system is affected by two things: centralization and the fact that the country is an archipelago.
Most quality health care is happening in Metro Manila. What you find outside is generally very basic, and even in 2nd-tier cities like Cebu or Davao they may refer you to Manila for something you'd think they ought be able to do.
With travel by boat being … rather slow, with vacation time not exactly ample, and the average salary being low I'm quite certain that affordable flights ($25 Cebu - Manila) are making ample difference in terms of what kind of healthcare situations are being addressed.
Another example is family members overseas. Many have to choose between providing for their family vs. being with their family. Affordability of flights can have a huge impact on their quality of life and ability to take part in the moments that many of use take for-granted.
Anyway, I have this feeling that you could justify just about anything if you dig for nth-order effects, but it doesn't change the fundamental point: a race to the bottom sacrifices everything that can be sacrifices. Environment is usually the first victim, quality the second, but safety trade-offs are eventually made too. At some point in a product category's lifecycle, one starts to wonder whether it's so degraded that it would be better if it didn't exist anymore.
Which means we don't know how much these things actually cost in real terms. We also don't know how to quantify potential benefits.
(And if someone says 'Well, that's subjective' - so is nominal market value.)
There's conventional profit-loss accounting, and there's everything else. It seems the everything else can accumulate to ecosystem-threatening levels, and conventional accounting will continue to pretend this is financially irrelevant.