cost sb an arm and a leg:to cost sb a lot of money (使)花一大笔钱
This ring cost me an arm and a leg.
我为这戒指花了一大笔钱.
Cost an arm and a leg表示"耗资不菲,付出过高代价",这仅仅是一个简单的比喻而已,用失去人体四肢中的"两肢"来比喻"付出昂贵代价".
overcharge:to make somebody pay too much for something 多收(某人的)钱 verb ~ (sb) (for sth)
Make sure they don't overcharge you for the drinks. 注意别让他们多收饮料费.
We were overcharged by £5. 我们让人家多收了 5 英镑.