There are many synonyms for the phrase "cost an arm and a leg". Some examples include "break the bank", "burn a hole in one's pocket", "put one in debt", "deplete one's finances", "be outrageously expensive", "be prohibitively expensive", and "cost a small fortune". Each of these phrases conveys the same idea that something is very costly and may require a significant sacrifice in terms of money or finances. It's important to remember when using these phrases to consider the context and ensure they fit appropriately with the tone of the conversation or written piece.