A priceless object has such a high value, especially because it is rare, that the price of it cannot be calculated.

A priceless collection of vases was destroyed. 一批贵重的花瓶被毁坏了。

figurative Her knowledge and experience would make her a priceless (= extremely useful) asset to the team. 渊博的知识和丰富的经验会使她成为队中的无价之宝。

extremely funny to see or hear

You should have seen the look on her face when I told her - it was priceless! 你真应该看看我告诉她时她脸上的表情——太好笑了！