The following is a strategy for getting the most out of your GPT trades with the AI.
Have you often been in annoying situations with the AI, wanting to sell off a resource, but getting only a pittance in return? Is silk really worth only 3 GPT?
It turns out that the AI also values these resources more than they are paying, but are limited to only giving away as much GPT as they are currently making in profit. One of the most striking features of this is the fact that the AI always seems be willing to trade all of its available GPT for your goods, whether it is 1 or 5. How would you like to make 7, 9, or 12 GPT regularly per resource. 
Well now you can! All it takes is a little patience, and plenty of extra goods, and you’ll be in bushiness.
The primary limit on an AI’s GPT trading is simply the size of their positive cash-flow that turn. There is a secondary limit based on how much they like you, but you don’t often hit it. In order to trade as much GPT as possible with the AI, you have to make sure that they have enough GPT to cover the transaction. How do you do this? Subsidize them!
Subsidies: If you gift GPT to an AI in negotiations, it will (as you might expect) raise their GPT in turn. Gift an AI GPT in small increments until their Available GPT stops increasing. You have just determined their trading limit, and they will now be willing to buy resources off you at top dollar.
Here is an example scenario:
Suppose I was trading with the Persians, and they are listed as having 4 Available GPT.
- I gift them 2 GPT, and their Available-GPT rises to 6.
- I gift them 2 GPT again, and now their Available-GPT rises to 7.
Where did the money go?
They are simply not willing to trade at 8 GPT; their limit is 7.
- I now sell them Furs for 7 GPT, and now their Available-GPT is at 1.
- I gift them 7 GPT, and their Available goes to 7.
- I sell them Rice for 7GPT, and their Available is down to 1 again.
- I see that I can only sell 1 more resource to them, Silk, and so I gift them 6 GPT. Their Available is at 7.
- I sell them the Silk for 7 GPT, and call it a day.
You are all probably noticing that throughout this entire affair, I’m only making a profit of 4 GPT while selling them 3 resources. I’m effectively giving them 2 resources for free! That is however, why it is called a subsidy. You don’t make a profit just yet.
Making your profit: For the first 10 turns of your trading agreement, you’ll be getting the short end of the stick. That now changes, because after 10 turns, you can cancel trade agreements! You are currently subsidizing the purchase of your goods at a cost of 17 GPT, but once the 10 turn limit expires, you can cancel these. Now, you are turning a nice healthy profit of 21 GPT from resource trading, not too shabby. The best part comes from the fact that you can do this with every civilization, resource sourpusses permitting. I have personally been able to break 100 GPT in trade-profits using this method.
Things to watch for:
- If your potential partner’s Available GPT is listed as 0, they are probably in deficit spending. Don’t bother trading with them, since you’ll have to subsidize them out of their deficit before negotiations can even begin, and you have no idea how much that’s going to take. 5 GPT? 10? You have no way of knowing, and it will be expensive no matter what.
- You yourself need a positive cash-flow before you can gift people GPT. To be on the safe side, set your science spending to 0% before entering negotiations.
- Always sell to the highest bidder. Search around for who has the highest trading limit, and choose to trade with them first. This will make sure that you get the most out of the sale of your resources. Trading limits correlate pretty strongly with the Civ’s attitude towards you, so you can be making 12-15 GPT per resource from your close allies.
- As nations become friendlier with you over time, their trading limits will begin to rise. If you suspect this to be the case, you can cancel your trades and renegotiate. Their Available GPT will rise in accordance with their suddenly reduced expenses, and will still be available to you for buying your goods. If, in the Persian example, after a while the trading limit rises to 8, you renegotiate to sell them the first two resources for 8 GPT apiece, but you will probably have to subsidize the third. No problem there, since subsidies are a 0 net-loss, so for the first 10 turns, you will just be making 21 GPT. Once that’s up, you can cancel the subsidy and come up to 24 GPT, making the whole thing worthwhile.
I do not believe this to be too much of an exploit, since the AIs have their own personal spending limits anyway, and are perfectly willing to pay that much for the resource. Furthermore, it seems stupid in retrospect to be selling your wheat off for 1 or 2 GPT. This tactic simply gets the AI to trade at their maximum trading level for as many resources as you have to offer. Now it might be theoretically possible to cause serious economic harm to a Civ when you cut off their subsidies, but that’s why this is called Aggressive Trading. You make them trade to their very limit.
Additional Uses for Subsidies:
1. Extortion: (Edit: This has not been thoroughly tested and appears to deliver rather poorer results were originally though.)
All GPT deals are restricted by the amount of Available GPT the AI has in stock. Subsidizing allows you to bypass this limit, and so opens up a new world of options, namely “Coercive Bargaining” or “Protection Fees”.
How to make use of this?
- Get a Large and Threatening(tm) army.
- Look for a weak Civ somewhere in the world, who you think might like to “stay in bushiness”, but might be too much hassle to “take care of” yourself.
- Having selected your victim, gift them enough gold up to their trading limit, to ease open their finances.
- Demand a repayment in kind, but for as large a sum as possible (you are limited to their trade-limit).
- If the first “trade” was not enough, try this again and again to see how much you can squeeze out of the little sucker. You may have to keep pumping them up with GPT so that they can afford your demands. (Tests show that AIs rarely if ever agreed to demands twice in a row. Yields are probably limited to 1 trading-limit’s worth.)
- After 10 turns are up, go ahead and cancel your side of the bargain, but their tribute will still be coming in.
I won’t even pretend this is not an exploit, since any sensible player would cancel their tribute to an AI guilt-free after 10 turns were up. This will theoretically allow you to soak your victim nations for a significantly larger chunk of change than you could normally, since a 0GPT in diplomacy can’t limit the tribute. The AI will faithfully send it’s moneys to you basically forever once the deal is made, ending only if one of you declares war on the other. The best part about this is that you don’t need to actually have any other resources than just a large military. Your relationship may suffer, but do you really care? If they don’t like paying, then you can just tax them like normal citizens of your empire (after you ‘assist’ their citizenship).
Edit: Many questions have come up as to “How do I cancel my deals?”. Due to some bug, it seems to be impossible to cancel a GPT loan in the diplomacy window with another Civ (it prompts to change the amount given, but then does nothing). To cancel these deals, you go to your foreign advisor (F3), and look under the “Active” section. This will show all current deals you have, and mousing over them will reveal how many turns until they can be cancelled. Click on a matured deal and you will be asked whether you wish to cancel it (y/n).