The characters combine to mean “[its] value connects [multiple] cities.” This paints a vivid picture of something so precious that its worth is not measured in money, but in land, power, and territory.
The soul of `价值连城` lies in the legendary tale of the 和氏璧 (Hé Shì Bì), or the “Jade Disc of He.” The story goes that a man named Bian He found a remarkable piece of raw jade and presented it to two successive kings, both of whom had their skeptical jewelers declare it a fake. As punishment for trying to fool the king, Bian He had both of his feet cut off. When a new king ascended the throne, he heard of Bian He weeping by his jade stone and, upon re-examining it, discovered it was the finest piece of jade ever seen. Later, this legendary jade disc fell into the hands of the state of Zhao. The ambitious king of the powerful state of Qin offered to trade 15 of his cities for it. This historical offer is the direct origin of the idiom, cementing the idea that some objects are so valuable they are worth more than entire cities. Comparison to Western Concepts: A common English equivalent is “worth a king's ransom” or “priceless.” However, `价值连城` is more specific and grandiose. “A king's ransom” is the price to save one person. `价值连城` is the price to acquire one object, and it's measured in land and political power, not just money. This reflects the deep cultural reverence in China for objects of historical significance, particularly jade, which symbolizes purity, nobility, and eternity.
`价值连城` is a formal and literary idiom. You would not use it to describe your new laptop, no matter how expensive it was. It implies rarity, historical or artistic significance, and a value that transcends mere money.