Quickleaf
Legend
I think that's something we'll eventually get into. I don't want this to dominate their time for the rest of the campaign, but finding that balance between what's happening everywhere in the world and how it affects the players and their assets is a unique challenge. But once the players have successfully laid the ground work for their Port, there has so be some semblance of conflict every now and then. It's D&D afterall.
The problem with the DMG's downtime rules is that it doesn't scale well, as evidenced in the links [MENTION=20323]Quickleaf[/MENTION] provided below. I don't want the running of a large port to be on the same scale of profitability as a pop-up stand on the side of a busy road.
I think that link I gave you answers the gold piece accounting question well by providing solutions to the strange economy implied by the DMG downtime rules.
However, if I was in your shoes that would just be one small fraction of the design.
Here's the direction I'd go in:
What makes a port desirable to PCs?
- Access to Rare Stocks. Befriend/attract/engage the right merchants and you may gain access to improved stocks from far afield. This might even include spells, magic items, masterwork arms, etc.
- Scope of Information/News. Similarly, invest in the right taverns/merchant houses/dockworker housing (or paying the right spies), and you may gain access to a greater/more international pool of news (i.e. your "Gather Information" skill – or whatever equiv. you use in 5e – gains a much greater scope).
- Ship Passage. Broker passage on a variety of ship to far-away locations...where you can meet the local monsters, kill them, and take their treasure.
- Naval Building/Actions. Assemble a navy which you can use to guard allied merchant ships against pirates, create blockades of enemy city-states, etc.
- Making Money. Some players may be interested in accumulating lots of gold. I've found this to be rare in 5e unless you're including ways to spend gold that aren't built into the system (e.g. using a homebrew Strongholds document).
Those are the main things of interest to PCs that are unique to a port. Maybe I've missed 1 or 2, so add those in.
Then you come up with some kind of rough ranking system (1-5) for each category, requiring X gp/time passed/quest goals accomplished to advance in rank. So you might have Access to Rare Stocks: 0, which basically means standard PHB equipment (modified according to culture/tech level), and then say, OK, if PCs pursue connections with a couple merchants, they can boost that to Access to Rare Stocks: 1, which you might interpret as some combo of low expense poisons, common magic items, exotic weapons, etc. I would just have a loose idea, since in actual play, I'd be adapting this to what the players actually decide, which NPCs they actually choose to engage with, etc.
Then with Scope of Information/News, you'd basically treat 0 as the local region, 1 as neighboring ports accessed through shallow coastal watercraft, 2 further ports accessed through shallow coastal watercraft, 3 further ports accessed through deep watercraft, 4 hidden, magically concealed, or hard-to-access ports, 5 very distant ports. This would affect the scope of what a PC learns by "asking around" or calling upon certain background features, and how broad their "early warning system" extends in the even of incoming threats.
Then set up a rough 1-5 ranking for the other categories.
IOW, rather than focusing on just the money side of things – focus on the broader reasons why players care about controlling a port. What can this do for us? Maybe we can make money...but that's not it's own end...why do we care? Access to goods, access to information, access to new places, and naval power.