The Great Monetary Collapse was a term used to describe several devastating economic disasters to affect the Ferengi Alliance during their history.
A notable Great Monetary Collapse struck in the year 9315 on the Ferengi calendar, and lasted for 150 years. This collapse was caused by Drek, a cobbler who made high-quality boots at low prices, thus bringing about a collapse in the slither eel hide market, which then spread to the entire garment industry and then the entire economy. (DS9 reference: Legends of the Ferengi)
Another Great Monetary Collapse struck in the 2350s, during which the Ferengi economy suffered rampant inflation and its currency had to be devaluated to compensate. Quark, then serving on a Ferengi freighter hundreds of light years from Ferenginar, was unable to save the money in his accounts back home. Later, he would compare the Collapse to the bombing of the Antwerp Conference in 2372. (DS9 episode: "Homefront")