pay an arm and a leg

To pay an arm and a leg means you have to pay too much money for something, much more than what it is worth. It can also be used simply to say you have to pay a large sum of money for something. Paying an arm and a leg is a way to exaggerate how much something is going to cost you, as a way to not only say it costs a lot of money, but figuratively it costs nearly everything you have.