Okay, the user wants me to translate "external costs" into Thai. Let me check the rules again. The first rule says to translate all content into Thai. The second rule states that if the content is already in Thai, leave it as is. The third rule is to provide only the translation without any explanations.
So, "external costs" is in English, so I need to translate it. The correct Thai translation for "external costs" would be "ต้นทุนภายนอก". I should make sure there's no explanation, just the translation. Let me confirm that there's no existing Thai text here. Since the input is in English, I proceed with the translation. No need to add anything else. Just output "ต้นทุนภายนอก".