‘A pretty penny’ is an American figure of speech. A ‘penny’ is a coin. In American money, it is worth 1 cent. (Approximately .317 New Taiwanese Dollars). ‘A pretty penny’ means it cost a lot of money.
我的車子送修，花了我好多錢。
A penny 在美國貨幣中只是 a cent（一分錢），一枚硬幣而已；不過 a pretty penny（漂亮的一文錢）卻是很多錢。這是美國式的修辭，一種比喻的說法 (figure of speech)。