The sultan of Brunei's wealth comes from oil and gas reserves. Theoretically this income belongs to the country but in reality it is used like personal income. The sultan was once the richest man in the world but is now 'only' something like 4th, thanks to a profligate brother, Prince Jefri. His wealth in the early 1990s was estimated at $40 billion. As an undisputed ruler he has access to the riches of his country.
The oil and gas fluctuate in value but earn him around $2 billion a year. His mind-blowingly lavish life-style do not appear to have diminished his reserves. He has done some good using his own money to prop up economies in other South Eastern Asian areas.