I live in Thailand as a foreigner. The reason for the price difference is to keep the fees low for Thai Nationals so every Thai can afford an opportunity to visit their national parks and historical sites. As the minimum daily wage is 325 baht, many families still have to save to visit a national park. I personally know many Thai families, who would like to visit Khao Yai, but can’t afford the cost (transportation, food, lodging & fee) of traveling to the park.

Rather than feeling insulted being charged a foreigner entrance fee, you should feel lucky to be able to afford and have the opportunity to visit this natural treasure of Thailand.