Opeks was a form of currency, accepted in the Cyrrhenic Empire and on Ribos.
Ribos itself was valued at ten million Opeks by the Magellanic Mining Conglomerate. Graff Vynda-K expected Garron would take six million for it, but they settled for eight million, including a two million Opek deposit which the Graff haggled down to one million.
The Shrieve captain was reluctant to accept a bribe of one hundred Opeks from Garron, until he raised it to one thousand. (TV: The Ribos Operation)