Officially, it's the Saudi National Bank that is getting burned. But, in an absolute monarchy, you might as well say it's Crown Prince MBS because there's no distinction.
There's a reason why Forbes does not even rank royals' wealth. It's a futile journey trying to decipher what belongs to the king and what belongs to the state. But, note that the king must spend a lot on social services (free healthcare, free education (both local and foreign) in Saudi Arabia, etc., to please his countrymen, lest they revolt.