The phrase "bought for a song" indicates that something was purchased for a very low price. The opposite of this phrase could be "bought for a fortune", which means the item was purchased at a very high cost. Another antonym for "bought for a song" could be "bought at market value", which means the item was purchased at a fair and reasonable price. "Expensive" and "overpriced" are also antonyms for this phrase. It is important to consider the value of something before purchasing it, whether it is bought for a low or high price.