The arm and the leg are important parts of your body. There are lots of things that you will not be able to do if you are missing an arm and a leg. When you buy something and you say that it cost you an arm and a leg what you mean is that you paid very dearly for it. It cost you a lot of money. It is also possible to say “pay an arm and a leg for something”. Another expression that means more or less the same thing is “pay through one’s nose”.
*The tickets for the one-day match cost Manju an arm and a leg. *I am not going to pay an arm and a leg for a piece of junk like that.