Antonyms for the word "marked up" could include "decreased," "discounted," "reduced," or "lowered." These terms suggest a decrease in price or value, rather than an increase. Another antonym for "marked up" might be "undervalued," which implies that the item is priced lower than its actual worth. It is important to keep in mind that the opposite of "marked up" can vary depending on the context. For example, in the world of retail, "marked down" might be a more common antonym than "marked up," as it suggests a decrease in price rather than an increase.