When shopping, most people look for bargains, but are often disappointed to instead find products that cost an arm and a leg — a lot of money.
This phrase uses exaggerated language to express how expensive something is. It takes so much money to buy it, you might as well be losing two important limbs. Similar exaggeration is used in the phrase highway robbery, where something is so expensive it feels like you're the victim of a crime. The concept is reversed in the phrase give my right arm for, where you're expressing that you want something so much you'd consider getting it almost worth the loss of an important body part.