The antonyms for the term "free charge" are numerous. These include "expensive," "costly," "pricey," "cost," "tariff," "toll," "fee," "duty," "wage," "amount," and "charge." All of these words are essentially synonyms for the concept of payment, emphasizing the fact that something is not free of cost. So, depending on the context and the industry, the opposite of "free charge" might be any of these aforementioned terms. In summary, "free charge" is a term that implies there is no cost, and the antonyms all relate to the concept of payments and expenses.