You will find that dive equipment in Thailand is quite expensive comparted to prices in the EU. In general I would say 30%-50% more. If you live in Japan why just just buy online and ship it to Japan, if I am not mistaken Japan has no import tax for water sports gear. Also you will not have to pay EU VAT since its shipping out of the EU. I buy almost all my gear online from the EU and ship it to the US (they too have no import tax on dive equipment). Last year I even ordered a rebreather from France to the US did not have to pay VAT in the EU or pay import tax in the US, literally saved like €2,000+
I recently bought about 25kg of dive gear from Spain to the US via UPS and I think it was €70 for shipping but I saved about $700 buying the gear from Spain vs buying it online in the US and maybe double that compared to buying it in SE Asia.