Almost every set ships in both English and Japanese. The Japanese box of the equivalent set trades well below the English one. Here is the gap across the modern era, widest first.
+252%
The median English box costs about that much more than its Japanese counterpart. A pair below the line is where Japanese holds value relatively well, or English looks comparatively cheap.
It is relative value, not free money. A Japanese box holds 30 packs to English's 36, pulls from a different card pool, and carries no English text. Importing adds shipping and customs.
Japanese sets also release roughly half a year ahead of their English counterpart, so the Japanese side often moves first. A premium that is narrowing or widening over time says more than the level alone.
Pairs are matched by hand across the Sun & Moon through Mega eras. Booster boxes only. Prices are the latest recorded for each box.