How profitable would sacking a major city be? Kings landing has been sacked before by Tywin, aerys went wild in duskendale, and there is the potential sack of old town coming up, which I imagine woul be comparable to the Mongols sack of bagdad.

In the first Sack of Rome in 410, Alaric of the Visigoths extracted 5,000 pounds of gold, 30,000 pounds of silver, 4,000 silk tunics, 5,000 dyed leather hides, and 3,000 pounds of pepper. While technically that was more of a ransom than the city itself being ransacked, it does indicate how much one could theoretically extract from a city.