Well I've been researching into ancient coins and currency. Particularly Roman currency. I stumbled into an interesting calculation. If D&D were played on Earth there could only be 10 to 20 high level adventurers. Four of five major dragons and a lich or two.
Look at it this way an Aureus, the gold coin of ancient Rome and equivalent to 25 Denarii weighs about 8 grams. The total world production of gold (for all times up to today) is 160,000 metric tons (see Gold Reserve). That allows us to mint 20,696,625,096 gold pieces. If a 11th level fighter requires 1 million XP the world's gold allows for 20,700 11th level fighters. But there has to be some money in circulation and some money in hidden treasures and more money with the dragons and vampires and liches. What about all those second, third, fourth and fifth level adventurers?? A fifth level party of 5 requires about 200,000 gp to get there. All this of course is including 20th century production with modern extraction methods.
What would an empire's bullion amount to in ancient times? Well according to one source Alexander the Great took 120,000 talents (aprox 4200 metric tons) of gold from the ruins of Persepolis. That's about 562,500,000 gold pieces. That's a lot of XP!!!! But if you take into consideration his army's strength of 40,000 men, how much does that leave for character progression? About 14k per soldier if evenly divided. Not so much when we realize we just pillaged the gold reserve of the Persian Empire!!!
So while one XP per gold piece sounds nice in D&D it isn't quite sustainable in real economic terms. Thank goodness the impact of such amount of bullion entering a city's economy isn't taken into consideration. Save the town from the orcs one week and wreak economic havoc the next by flooding it with the dragon's bullion.