Currently, there's two kinds of resources in War Worlds: goods/food and minerals. Food is used to sustain your populations (not enough food and the population in your colonies starts to drop). Minerals are used to build things.
But one of the things I want to add to the game is the ability to trade. One of the things I like about the game Pocket Empires is the global market place where you can go to buy and sell units. It's based off a modelling of a stock market where you bid for items and the price fluctuates as biddings increase or decrease. I really like this idea and would like something similar for War Worlds, but in order to do that I need some kind of global currancy.
Earning Money
The first question, then, is how do you earn money?
Currently, I'm thinking a simple tax on your population will do. That is, each hour (or so) you earn $x per unit of population across all of your colonies. So for example, you might have 4 colonies with 600, 700, 800 and 900 population each. Let's say you earn $0.33 per unit of population per hour. That would mean you get $990 per hour from your colonies.
These numbers are just a rough estimate, of course. In the future I could even make it more complicated like if you're got a "happy" population you'll get more money etc. But for now I think a flat rate is the easiest to understand.
Trading
To actually trade, I imagine something similar to Pocket Legends as I described above. That is, there's a "global" marketplace where you can put up a fleet from your collection for sale at a given price. People put bids in at a certain "buy" price, and trades are made when the prices match up.
To begin with, I can see trading food and minerals as well as ships and possibly buildings.
One problem that I can see is that if you buy ships from another player, how do they get to one of your stars? If they're "instantly" transported from the player's space to your space you could see it as a way to move fleets across space really quickly. So for example, if I want to transport ships between my colonies quickly I could make a sell and then a buy in quick succession to move ships to a different star without actually having to do anything.
Another issue is that if I'm being attacked by another player, I can just buy backup ships in the market and have them appear in my star system immediately. That doesn't seem fair when in reality I should have planned for the attack better.
So I think the obvious solution is that when you buy ships, they appear outside your star system, some distance away and you've got to wait for them to arrive. I think that would solve most of the problems I can think of with an "instantaneous" transfer.
Spending Money
One problem with the above is that money would be added to the universe but never destroyed. Trading simply causes money to change hands, but if money is being made from "nothing" then it needs to go somewhere or you'll just end up with hyper inflation (if you look at food, it is used by populations for sustenance, minerals are used by ships which get destroyed when they fight, etc).
So what I'm thinking is that moving fleets between stars will cost "fuel" (which costs money). So for example, a 20 parsec jump might cost $1 per ship, 100 ships costs $100 and so on.
This has the side benefit of somewhat restricting your ability to expand infinitely. You can't just fly your ships out to the deepest depths of space, because all that fuel is very expensive. Well, it'll still be possible, but hopefully the opportunity cost will make it so that few people will actually bother.
We could also have some ships that are more fuel efficient than others. So it cost less for some ships to move between stars than others. For example, moving a colony ship might be very, very expensive making it more likely that you'll colonize multiple planets within a star system than lots of individual stars. That would also mean you're more likely to "cluster" your colonies among close stars, rather than spread out really far. That could be a good thing or a bad thing, I suppose, depending on what the desired behavior is.
Real Money
So that brings us to the last point, "real" money. I was thinking that the primary in-app purchase would therefore be in-game cash. If you can buy in-game currency with real money, then it would essentially be a short-cut if you can't wait for your colonies to pay their taxes. From what I can see, this kind of in-app purchase is usually pretty common, and I don't think it unfairly advantages players who are willing to spend money over those who aren't (you just have to wait longer).
Conclusion
So that's how I'm thinking cash will work in-game. As with all things, this is all quite subject to changes and tweaks.