The darsek was the main unit of currency used in the Klingon Empire.
In 2153, the Tellarite bounty hunter Skalaar should have been paid 9,000 darseks for the delivery of the Human Jonathan Archer, but was later fleeced of 3,000 darseks. (ENT: "Bounty")
In 2370, on Maranga IV, Alexander Rozhenko asked his father Worf for 50 darseks to see the mummified head of Molor. (TNG: "Firstborn")