This is the practice of charging different rates for Thais and foreigners. As an example, entrance fees for national parks are 20 baht for Thais but 200 baht for foreigners. Many organisations in Thailand, both public and private, practice two-tier pricing. Common examples are museums, palaces, zoos, aquariums, shows and national parks.
There are two viewpoints on two-tier pricing:
Those who support two-tier pricing say it is only reasonable that Thai people who are not as wealthy as foreign visitors should be able to see attractions in their own country at a price they can afford. If Thais had to pay the same price as foreigners, many would not be able to see attractions in their own country. Furthermore, Thais pay taxes that subsidise attractions such as national parks and therefore should not have to pay the same as foreigners who do not pay taxes.
Those against say two-tier pricing is racial discrimination and cannot be justified on any grounds. It reduces the number of foreign visitors to attractions that practice two-tier pricing and therefore does not actually earn more money. Furthermore, it gives foreigners a negative impression of Thailand that may actually reduce the number of tourists coming to the country and therefore cost Thailand far more money than is gained from two-tier pricing. Also, foreigners do pay tax as they are much more likely to frequent establishments that are VAT registered.
Both arguments are valid and the issue is a cause of contention. In our opinion, the biggest problem with two-tier pricing is not the amount of money involved - the prices will still seem cheap to most western visitors. The real problem is the message it sends to ordinary Thai people. If it is okay for the government and businesses to charge foreigners more than Thais then it must be okay for everybody else to do the same. Hence tuk-tuk drivers, shopkeepers, market stallholders, etc, will often quote higher prices to foreigners than to Thais. It is a widely held assumption that all westerners are wealthy and should pay more than Thai people.
The places practicing two-tier pricing are clearly aware it is contentious. They give this away with their entrance signs. They write the foreigner price in English with Arabic numerals. They write the Thai price only in Thai script - including the numerals. These days, Thai numerals are rarely used and have been almost entirely superseded by Arabic numerals. The use of Thai numerals to display two-tier prices is a clear indication of concealment.
Long-term residents can often avoid paying the foreigner price. If you can show a Thai driving license, work permit or tax slip you will probably get the local price. Even a sprinkling of Thai language might get you in for the local price. Also going in a mixed group of foreigners and Thais may result in everybody getting the Thai price as the people on the gate are often embarrassed to discriminate between people in the same group.
Whether you think two-tier pricing is justified or not, it is a fact of life in Thailand. It is an individual choice whether two-tier pricing bothers you enough to justify boycotting places practicing it. Is it really worth missing a beautiful national park or other attraction over a few baht on a point of principal?