When shopping, most people look for bargains, but they're often disappointed to instead find products that cost an arm and a leg — a lot of money.
The phrase an arm and a leg uses hyperbole, or 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 it is suggested that something is so expensive it feels like you're being robbed. The concept is reversed when people say, I would give my right arm for (something): Here, they're saying that something is worth paying dearly for — even losing an important body part.