My player's characters are all 3rd and 4th level. They've gotten the idea to build and open a bank. They've saved up and purchased a building in the merchant district of the campaign's main city. They're hiring a blacksmith to construct a vault, and have hired a halfling NPC to be the bank manager, and former city guards to be security. They're not high enough level to start their own realms obviously. Everyone is having fun and they are still going to go on adventures. What I'm having trouble with is how to handle the players gaining income from said bank. Any suggestions?
My first thought was to treat it as a mercantile investment. Which average a return of 3% of their capital cost monthly, or you can randomize using Coins & Commerce (may be Patreon content?).
That's a little abritrary in this instance though as their own capital costs are simply the building purchase, vault and I suppose fixed payroll costs. (And, by the way, I hope they're coming up with security over and above vault and guards. Any self-respecting thieves' guild would be all over that.) It doesn't cover leveraging deposits to make loans, which sounds like the Money Lending line in investments, but using other people's money. Which sounds like an exploit until you remember you have to keep enough on hand to cover withdrawals and prevent a bank run, but how to model bank runs I have no idea.
So I think the thing to nail down is what does "bank" mean to them and to you. Specifically, does it include fractional reserve lending or no? If it's safe deposit only, that's a Safe, Business Establishment investment, which is straightforward, but maybe more boring and less lucrative than they might expect. If they're making loans with their own seed money, that's Money Lending and still straightforward, with variable risk and return.
If they're making loans with other people's money that's awesome, but it's where things get complicated.