I think the bigger question is "What in the game world costs so much that I need to spend a chest full of gold to buy it?"
You have here a question that is asked in every RPG, and yet every RPG to date simply takes the easy way out - you have unlimited gold carry potential for trade. This means that you can sell a private jet to joe schmoe with the infinite pockets.
What if you took an entirely different approach and considered how this would work in the real world (even 1000 - 2000 years ago):
The average land baron doesn't walk around with much money. Aside from the potential to be robbed, gold and silver (especially gold) has a substantial weight value attached to it. Paper money weighs a lot as well (anyone who has lifted a duffle bag of greens knows what I'm talking about). But the average person doesn't have or need that much money in one spot at one time.
To put it in game terms, why would the player carry $10,000,000 in gold coins to a blacksmith to have a sword made? Seems like a character with that much money would have a castle, some servants, and probably a wagon to ferry the gold.
So, then, why wouldn't the character simply have a servant order the item and transfer the gold?
This opens up three layers of economy:
1) The average (common) economy with little money. This is food, simple potions, basic weapons, and other sundries you would expect to find in day to day life. These are relatively inexpensive items that are expected to be purchased in small quantities.
2) The mass production economy with large quantities of money and relatively cheap items. The players who like to buy 100 health potions at once would fit in this category. After all, you probably won't go a gas station and buy 100 bottles of soda. Likewise, there are bound to be laboratories, larger smiths, etc that produce goods in enough quantity for cities, armies, and empires. Instead of selling one or two items directly to the player, the player must go through a broker and buy in quantity. These are then shipped to the player's destination of choice (castle, manor, house, tent, ditch, etc).
3) The luxury economy with rare and extremely expensive items. These items would be difficult to track down and require a chest or two full of gold and gems to buy. While the product is probably small enough to carry, there is no way that the player could possible carry that much gold to make the purchase. In other cases, the item may have to be delivered (catapult? Bronze Statue? Chevy Blazer?).
To facilitate the second two economies, the aforementioned bank system could work, where the player deposits gold into a banking system that tracks it (and eventually their own estate?). Instead of instant delivery, the player could expect delivery in x amount of time (either for transportation, production, or both.
Obviously I don't know how the game is going to play, and how much the passage of time matters, but this could add another depth to the game where your gold shipments could be lost or raided, and hiring guards to protect the shipment a way to balance the economy a little more. If people want to live like land barons, they have to spend money to protect their holdings.
That's just my thought.
