Spending money can also become a character trait that influences other parts of the game. If you are known as a penny pinching scrooge who only buys the cheapest of items that serves to meet the bare minimum of their needs, then the game world can respond to you as such. If however you throw money around, then the world will also respond as such. Money attracts money. Spending 'extra' over and above what is strictly needed can come with benefits. Attracting different classes of NPCs as followers or something.

I really don't see the point of "two currencies" as you've described it. If you have a ton of money then purchasing stuff to satisfy basic personal needs should be as trivial as access to services for those needs.

That's not to say you couldn't give the player incentive to not carry around millions of gold coins. First of all, it would make sense that carrying around a million gold will make you a target for thieves. But also if you simply add weight and capacity limitations to money (and other items) then that gives the player another reason to consider how much of anything needs to be on his person or transported with him. Money he doesn't carry could be in a bank account (or multiple accounts) and transactions involving large amounts could be done through drafts, contracts, agreements, or whatever. It's possible that dealing with financial institutions and their (or government) regulations opens up a few other possibilities for gameplay as well. Presumably, banks aren't going to be everywhere you go and they won't necessarily all deal with each other so some planning in that regard might be necessary.There might also be an avenue to explore if the player is to be dealing with less reputable clients and somehow needs to complete the transaction without raising suspicion.

If you're going to have more than one currency then I think it should be because you're adding international trading into your game, not because you want to constrain the player.

One of the old might and magic games had two currency gold and mega credits. Mega credits were only used upgrade your castle. But then that was a two currency system.

You could just scale costs accordingly. Rare valuable items might be worth as much or more than your entire trade guild . That would give the game a little more sense of realisim. It might be that a single drop from the fountain of youth you could trade for a castle and its surrounding lands.

Or have a guild vault were all your business earnings go into and you can only take a percentage for yourself.

Scaling one set of items up or down seems like the easy solution. 1000 bushels of corn could cost 1 gold, while a good sword could cost 10 gold. It's sort of realistic in the sense that a horse and fully articulated plate mail were very expensive in the medieval era, so only nobles could afford the good stuff.

Just a sidenote:

Bronze weapons didnt exist during meadieval times (altough often included in games), and during the age they were used (before 800 BC) they were shortswords and daggers mostly, not full sized swords as bronze is too weak for this.

Also as another side note. What suilman said is also why the more advanced ancient bronze weapons were leaf shaped. The shape of the leaf gave the flimsy weapons added strength.

Scale the responsibility of the wealthy to employ and delegate responsibilities of market. Scale up production, increasing the risk of meeting mass demand instead of smaller scale market sales.

I say you've got to have plenty of short term, mid term, and long term goals. If your players are amassing hoards of wealth, you don't have enough long term goals. Throw in some epic construction projects or something, but there should always be something they want to spend their resources on.

Just a sidenote:

Bronze weapons didnt exist during meadieval times (altough often included in games), and during the age they were used (before 800 BC) they were shortswords and daggers mostly, not full sized swords as bronze is too weak for this.

What is stunning is that bronze is much more expensive nowadays than iron. And even though it is much less durable, it is still used for some very expensive tools (garden tools, for prestige) and machinery (marine propellers, because of corrosion).

On the other hand, for nearly 1000 years after the discovery of iron, bronze was affordable to the common man and iron was not (because there were three coppersmiths in every town, and making iron was kind of "sorcery"). The tales of magic swords that kill dragons (Gram) or cut through massive stone (Durandal) made by magicians and dwarves from meteors and who knows what else testify for that. Producing iron was simply "magic", and it was only affordable for the rich.

Similar story for mirrors. Symbol for wealth until the 19th century, now there's one on every kid's bicycle.

Getting rather off topic, but it isn't really till the very late medieval period that you really see all that many iron swords surpassing bronze swords in length. In the early medieval period you still saw a good amount of bronze weapons in use and production, it was merely that good iron was preferred for its quality. However, a good bronze sword can generally trump a cheap iron one, and we have many examples of bronze swords being longer than much of the early iron swords.

Thousands of years of history over hundreds of cultures, all producing weapons with their own take on things. Very hard to generalize too much about it without a few points popping up to contradict what someone says. (Just look at bronze swords and their riveted handles vs tangs. We can see them swing back and forth between the two methods for a few centuries, while iron swords are almost universally tanged designs of one style or another.)

Just like a special adventure high protein/nutrient chocolate bar would cost you more then a standard snickers in real life, perhaps adventure gear is more advanced and also more expensive? Perhaps it's illegal or rare from some way, like traded into the game using far away trade routes. Meanwhile, the common stuff we buy every day is perhaps not cheap, but still costs a lot less.

Example: a magical plaster that heals infections is awesome for an 16th century adventurer and if such a plaster would have been available on the markets back then, it would have been worth a fortune.