Antonyms for the phrase "tidy sum" could include terms such as "paltry amount," "meager sum," "insufficient funds," or "scanty sum." These words convey the opposite meaning of a tidy sum, which usually implies a large amount of money or a significant sum. The term "paltry amount" suggests a small amount, often not even worth considering. "Meager sum" and "scanty sum" also indicate a small amount, while "insufficient funds" implies that the sum of money is not enough. Overall, these antonyms emphasize the opposite of a tidy sum and convey a sense of financial constraint or poverty.