There are several rumored reasons why the Thai dual price system exists. One is that the average foreigner is assumed to have more money than the average Thai and can afford to pay more. Others say that because foreigners don’t pay taxes, it’s a way to cover maintenance costs at government operated facilities, such as at national parks.
The Thai dual price system does exist in some forms of public transportation, popular travel destinations such as temples and national parks, and sometimes at touristy markets.
These price differences are typically found at places that are visited more by tourists and generally less by expats.