I just wanted to kind of compare apples with apples. The two jackets you bought at the end of the sixties were probably both high quality items (selvedge denim, produced in the US perhaps).
The 'cheap' denim trucker jacket is not produced with selvedge and neither in the US or EU, while the leather one is.
So, yes, the multiple seems to have come down from 12.5 to not even 4 between denim and leather. The reason for this is probably that relatively speaking hardly any denim is produced anymore in the US/EU and hardly any jackets are made there. So the price of such a denim jacket has risen overproportionately, while the leather and leather jacket industry is still comparatively active.
And IMHO the reason for the high prices in Japan besides the high quality materials and the high standard/costs of living/wages is that the Japanese market is very homogeneous and then relatively huge. That's why so many items are sold out so quickly.
While in the US and Europe only a very small subset of consumers is interested and willing to pay for these kind of items (perhaps 0.2%- just a guess), in Japan there are many more (more like 5%; also guessed).
And then there's the phenomenon that people often will perceive an item to have a higher value if it's more expensive. We know this from the French and Italian brands.
Very very happy to live in a society with "inefficient employment laws".