I agree with Korjik. I would start at the end of what you want to allow them to have so the focus of the campaign is your players adventuring, not running a business. But if you really want to figure out some kind of system, then start at the end (what the profit to be generated?) then work your way backward.
So here's a step-by-step process:
1. Determine the formula that you'll use. I don't know what the Middle Ages used as an accounting system, so I'll just base if off a more modern one. So here's a formula for you:
Gross Sales - Expenses = Gross Profit (Loss) - Taxes = Net Profit(Loss).
2. Net Profit / Loss. If you want to reward your players for their investment, then determine a gold amount that their Net Profit will yield and also make it a percentage of the overall Gross Sales. So, let's say that you want your players to earn 5,000 gp and you want it to represent 10% of Gross Sales which will be 50,000 gp. So now your formula is going to look like this.
Gross Sales 50,000
-Expenses ??
=Gross Profit ??
-Taxes ??
=Net Proft 5,000
3. Determine Taxes as a percentage. So this has to be arbitrary, but the taxes is taxes to the paid to the government, tithes to the Church, and any other entity that is entitled to payment, but doesn't contribute to the business. Let's set this at 25%. Now to work backward, you must take your Net Profit / 75% (1 - 25% taxes) and that equals your Gross Profit. 5000/.75 = 6,667 gp. So now we have this:
Gross Sales 50,000
-Expenses ??
= Gross Profit 6,667
-Taxes (25%) 1,667
=Net Profit 5,000
4. Figure Expenses. Expenses is easy. It's the difference between Gross Sales and Gross Profit. 50,000 - 6,667 = 43,333 gp. So what does 43,333 gp in expenses mean? It's salaries for all the miners, it's paying for guards to provide security to the mines, it's buying equipment that constantly gets worn out and needs to be replaced, it's cash bonuses to keep the miners' morale up and to keep them from pocketing whatever it is the players are mining. It's hiring alchemists (quite expensive) who will separate your precious metals from the ore in a safe / magical way. It's the hiring of assassins to kill off rival mine owners (if the players are looking at expanding). It's the hiring of prospectors to look for new veins in the area while providing security for them with the use of rangers. It's the payments of bribery to a circle of druids who are "uncomfortable" with the land being mined. And so on.
I created a spreadsheet for you so you can play with the numbers yourself. Enter the numbers in the green fields and have fun!
Happy Gaming!
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