This question can go in any number of ways. For the easiest question, I’ll give you the easiest answer: How much in coinage should the guy walking down the street be carrying? I typically assume that a person carries about what their daily wage would be when they are out and about. We’re talking about factory workers or craftsmen on their way to lunch or the bar, etc. Average guy makes 10sc p/day, so average guy has 10sc in his average pocket. Go after a powerful enchanter who makes 300+, and he’s going to have a lot more on him as “spare change” than the brewer.
But what I was really thinking about was more along the lines of how much would realistically be in the treasury of a local lord, baron, king, whatever. Let me air out my bias - I think it is very important to expect that most nobility (the ones who are actually owning and leasing or managing lands) should not have a lot of easy money laying around. OK, from a peasant’s perspective, it would be a huge amount of money, but not really. Example - Look at the annual report of any government or corporation. Look at their cash line and compare it to their revenue/sales line? Just because you pull in $5B annually, doesn’t mean you have $3B sitting around. So where’s their wealth? Well, the queen of England use to be considered the wealthiest woman in the world. What changed? They stopped counting the things that “the Queen” owns, but the Queen herself really couldn’t sell as they are national treasures. Yeah - She went from what could probably be $20B+ to a paltry $500M. That’s what I’m talking about. The Queen has palaces, jewels, art work, probably a stable full of fine polo ponies (I’m shooting from the hip here), yachts, farmland, etc. ad nauseam. OK, she could probably sell the jewels, but the rest of the stuff isn’t really saleable, certainly not easily.
So let’s assume - Your fantasy world is ruled by nobles. They control the “countries”. Their wealth may be completely mixed up in that of the government. In any case, just because they take in taxes of 1.2Msc in taxes annually and their own lands provide another 300K, how much of that is in the treasury (to be robbed)? Come on, we all knew that the only reason anyone really cares how much is there, is because they want to know if it’s worth knocking over the treasury. Here comes the big cop out answer - It depends! Seriously, if taxes are all due on April 15, then on April 16, the treasury is going to be looking pretty darn good! If taxes are paid in monthly, then you’d still see some far better piles once a month. But there would have to be decent level of coinage just laying around at the end of the month or the government would be borrowing massively. Yeah, I know, that sounds normal to us all because of our modern times, but it is unreasonable and unsustainable.
So what’s laying around? First, you have to know how centralized the treasury is. If the army and navy each have their own treasuries and so do some other folks, then the central treasury is going to be pretty thin. Then again, if the soldiers have to go to the central treasury to get paid, then you know all the cash is right there. Let’s assume that most of it is one place. It’s kind of tough to figure out what the current states and cities actually have. They don’t have an SEC watching their every move like corporations do, so they aren’t as transparent. (I know, you’re shocked, right?) Let’s back in: We’ll assume that the government collects 1,000,000sc (that’s silver coins) in revenue every year, because it’s a nice big easy number. We will therefore assume that they spend right around 1,000,000sc every year (or close enough for government work). Assuming a 12 month, 365 day year, they collect over 83,000sc monthly and spend about 2,700sc daily. If that’s it, you can guess that on the first day of the month, they have 83-84K, less 2.7K per day thereafter. But that assumes there is no cushion. Modern governments seem to have nothing but IOUs in their treasuries. Seriously, one state I looked at had >$65B in revenue and <$12M in cash spread around their agencies. That’s less than 2bps (or <0.02%). You’ll need to determine your fantasy gov’s cushion, but 10% of annual does not sound completely silly, especially if it is comingled with the noble’s funds. How big would that be? So now this example we have shows around 200,000sc at the beginning of the month. By the way - that’s more than four tons of silver and takes up almost 60 cubic feet (or a box 4’x4’x4’ assuming it was all melted into one huge cube - coins really don’t stack that closely together). The monthly 83K amount is around 3.5K lbs and 25 cubic feet. That’s a lot of coin. I know we used silver, but you have to assume that the savings you get from gold would be offset by the extra weight and space you’d need in copper.