If something is said to ‘cost a pretty penny’, it is presumed to be expensive (it costs a lot of money). This isn’t usually in the present tense (this costs a pretty penny); instead, you are more likely to hear it used in the past or future tenses. You might hear someone say that something ‘must have cost a pretty penny’, or that something ‘is likely to cost a pretty penny’ or ‘will cost a pretty penny’.