By far, the most valuable spice in the world today is saffron. The dried stigmas of the saffron crocus plant can be purchased as saffron threads. The reason for the very high prices is the expensive harvesting costs. The saffron crocus blooms once a year for two weeks. The pistils, of which each flower has three, must be painstakingly harvested by hand in the morning just after the opening of the flower, otherwise, the precious flavor evaporates. For two pounds of saffron, one needs at least 150,000 flowers or an acreage of approximately 10,750 square feet.

Source: Piccantino: World of Spices