Saffron is the most expensive spice in the world because it must be harvested entirely by hand.
Why it is true
Each saffron flower produces only three stigmas, and it takes about one hundred and fifty thousand flowers to produce a single kilogram of the spice, making the harvesting process extremely laborious and expensive.
Source: Agriculture
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Related facts
Saffron is the most expensive spice in the world, measured by weight.
Saffron is the most expensive spice in the world by weight, requiring about 75,000 flowers for one pound.
Vanilla is the 2nd most expensive spice in the world.
Vanilla is the only widely used spice that comes from an orchid.