In modern Norwegian folklore, mong is the term used for 5,400,000,000 Norwegian Kroner. In other words, one mong is 5,4 billion 1987-Kroner, roughly €70 million.
How did it come about?
In the 1980's, state owned oil company Statoil started building a giant refinery at Mongstad outside Bergen. The construction didn't exactly go according to plans, and in 1987 then Statoil CEO Arve Johnsen was forced to announce that the budget had been thoroughly smashed. The project turned out to be one mong over budget, sending Mr. Johnsen headlong into an early retirement after 15 years at the helm.
At the time, about 50,000 houses of reasonable quality could have been built for a mong.
After the infamous announcement by Arve Johnsen, Norwegian media quickly coined the term as an unofficial monetary unit, using it as comparison for budget overruns everywhere.
Even today, the slang still appears from time to time, especially in connection with large government and defense budget overruns.