Much of the "cheap retro games" from Japan are just the games that were more popular there like Paper Mario 64.

It's hard to really properly track these things but over the three trips I've made to Japan over about 12 years. Id say the price rises have been entirely in line with currency and retrogaming inflation.

I.e. I'm not sure it was every as good value as people thought.

I did buy quite a bit over a decade ago but again those were Japan only carts (that wouldn't even run in PAL without a mod chip but would run NTSC-U).

That said it is so much more touristy now I'm sure any arbitrage opertunity would be sweeped away same evening.

Japan is now also making domestic only console versions (at least for switch 2 and I think with PS5 on the cards).

Again this might lead to people thinking consoles are cheap in Japan but these are Japanese only consoles designed to revive the economic doldrum they are in.