The phrase "arm and a leg" is commonly used to describe something that is expensive or costs a lot of money. Antonyms for this phrase could include "inexpensive," "budget-friendly," or "cost-effective." Instead of breaking the bank, you could say that something was "affordable," "inexpensive," or "reasonably priced." Another antonym could be "free," meaning that something costs nothing at all. Overall, there are many ways to describe something as not costing an arm and a leg, including using words such as economical, bargain, or good value.