it's the link with money that I found interesting, $100 for the american units (I mean American book have problems in both system of units, aerospace, military and engineering for science tend to work in metric) and $30 for the SI unit only.

Because any competent engineer should be able to work with both interchangeably.

And almost all of the world uses SI units, so why would you expect anything else in the international edition?

I.e, all you've given us is a reason to only ever use SI. Which isn't really what your intention was, was it?

(I'm aware non SI is useful in some contexts.)

Which contexts?

I used to be a physicist so voodoo around constants so that we end up mass having energy units was common but I cannot think off my head of problems better where imperial units are easier. There are certainly, I am genuinely curious which.

Well, it's easy to remember that the speed of light is roughly 1.8 terafurlongs per fortnight...

I thought it was a joke but it is not. And "roughly" here is actually "very close to".

Now I know what imperial units are useful for, thanks.

If you ask any competent engineer in Australia to work with imperial units, they will go whaaat.

If you ask any competent engineer outside the US to work with imperial units, they will go whaaat.