Dual Pricing is Illegal in Thailand
Dual-pricing, having a different price for farang (foreigner) and Thai for the same goods or service, is technically illegal in Thailand. Even national parks owned by the state charge higher prices for foreigners, often by a factor of 10. Look carefully and you will see that the notices showing prices have two columns with the prices for foreigners in Roman numerals and the prices for Thais in Thai numerals.
Sometimes, an expat can get in for the much reduced Thai price by showing a Thai driving licence or work permit. But that is getting rarer. Speaking a little Thai and smiling used to help.
Thais stopped using their own numerals decades ago. You will notice that prices in street markets are now in Roman numerals. Apart from some government documents, you won’t see the traditional numerals anymore.
Restaurants often have two menus: identical except for the higher price (sometimes more than double) charged to foreigners. Look carefully and you will see Thais check their restaurant bills item by item. Take a tip from them.
Although future governments say they might outlaw dual pricing, I doubt it. Private hospitals have always charged higher prices for foreigners. The same may apply to the cheaper state hospitals eventually.