“We shall now speak of the revenue which the grand khan draws from the city of Quinsai and the places within its jurisdiction…In the first place, upon salt, the most productive article, he levies a yearly duty of eighty toman of gold…each toman equal to eighty thousand gold florins, consequently amounting to six millions four hundred thousand ducats…” 
The Travels of Marco Polo, Chapter 69, ‘Of the Revenues of the Grand Khan’