There was an old Vulcan saying, "isik for your thoughts?". According to Michael Burnham (during the mid-23rd century), no one knew what an Isik was or what the expression meant exactly. (DIS: "Despite Yourself", "Will You Take My Hand?")
In 2370, Alsia, a con artist, had apparently gotten word that the Vlugtans had accepted her bid for mining rights in the Vlugta asteroid belt. But according to Alsia, they wouldn't finalize the transaction unless she commissioned a study on the effects the asteroid mining would have on intersystem navigation. Because all her money was committed to the bid, she couldn't afford the study, which she claimed was 10,000 isiks. Once she was able to begin mining, the investment payback would be ten times the original investment, or 100,000 isiks.
Martus Mazur offered Alsia the profits from his own venture, Club Martus, to fund her venture. Alsia took his money and never returned to him to report the results of her bid. After Mazur was arrested for unrelated charges, Alsia later approached Quark with the same scam, which was revealed to Odo, and got her arrested.
Quark later offered to have Mazur released from the holding cell. Mazur then asked Quark to give him 2,000 isiks so that he could book passage off of the station. Quark countered by offering to loan Mazur 500 isiks so that he could book passage aboard a cargo ship. Mazur countered by asking for 1,500 isiks, because he had "expenses." Quark upped his offer to 600 isiks, and Mazur countered with asking for 1,200 isiks, stating that he still had his dignity. Mazur later reluctantly settled for 800 isiks. (DS9: "Rivals")