The phrase "cost a fortune" refers to something that is very expensive. There are several synonyms for this phrase such as "break the bank", "burn a hole in your pocket", "cost an arm and a leg", "exorbitant price" or "make a dent in your wallet". These phrases are commonly used to describe the price of luxury goods or services that are beyond the budget of most people. It is important to be aware of these synonyms as they can add more flavor to your writing and make your communication more effective. Using these phrases will also help you avoid repeating the same words over and over again.