The Immense Wealth of the Persian Kings
[The Persian quisling] Tiridates led Alexander [the Great] into a large building behind the palace of Xerxes [at Persepolis] that served as both an armory for the royal bodyguard and a repository for the king’s wealth. Diffused light filtered through a series of openings in the roof above and washed gently over the tons of gold and silver bullion that had been neatly and methodically stored there. Within the treasury building were 120,000 talents of bullion, the largest single concentration of wealth to be found anywhere in the ancient world.
Darius I had imposed a tribute of precious metals in addition to a tribute of goods on his satraps and on the subject nations of the empire. Instead of converting that tribute into coins that could then have been put into circulation, Darius and his successors had it melted and then formed into ingots of gold and silver. The bars were stored in the palace treasury, and when the kings of Persia needed to finance particular projects, wars, or adventures, the precious metals were cast into coins. It was Darius who had introduced the coining of money into the empire; hence, the Persian coin became known as the Daric. Until that time, the empire had been administered largely on the basis of barter.
Successive generations of Persian kings had dipped into the treasury and spent vast sums on themselves. Over the years, they had spent great amounts on administering and expanding the empire and had dispensed large sums in fighting, hiring, and bribing the Greeks. Yet no matter how much money the kings spent, every year at the New Year ceremony more came in to replenish and add to the royal coffers. In the treasury building at Persepolis, Alexander was shown the full measure of how wealthy the Achaemenid kings of Persia had been and how wealthy he had now become.
— John Prevas, Envy of the Gods, pp. 18-19
For comparision, Alexander started his invasion of the Persian Empire with a war chest of only 70 talents of gold.