All trades and actions occur in the same turn. Essential is the first trade, you HAVE TO pay more (the more, the more profitable) for the first resource than the AI would be willing to give it to you. In this example, our AI has Iron (Resource 1) and Furs (Resource 2) for you to buy. So we're going to buy Iron for 500 GPT as first trade. It will all pay out.
Assume, for simplicity, that 500 GPT equals 10'000 Cash. Depending on difficulty, attitude and former trades, this might be different.
Assume also for this example, the AI is willing to give you Furs for 30 GPT
|Trade Number||AI Gives||You Give|
|1. Buy first Resource for GPT overpriced||Iron||500 GPT|
|2. Buy back GPT from Trade 1 with Cash||500 GPT||10'000 Cash|
|3. Buy back Cash frome Trade 2, also "buy" Resource number two within this trade.||10'000 Cash + Furs||500 GPT + 30GPT = 530 GPT|
|4. Break Trade Road|
Note that, as long the AI is generally willing to pay GPT, it can offer the 500 GPT in trade 2, even if it had 0 GPT or low GPT to begin with. This is why you set up Trade Number 1, to make the AI's income statement look big
So what did we gain from this when we pillage trade route in step 4?
Trade 1 gets canceled by pillaging trade route. 0 Cost.
Trade 2 doesn't get cancelled since no resource involved. You receive 500 GPT.
Trade 3 does get cancelled, hence Resource Number 2 is required imho. This is the "Buy back your Cash" Trade. You don't pay GPT and got your Cash back from Step 2.
Net Income from those trades: 500 GPT.
Limitation in this example: You have to got 10k Cash on hand to make Trade 2 (or better 12k Cash on higher lvls).
The really interesting question now is, what happens to an AI that had no GPT left over to begin with, and is now forced to pay 500 GPT? The same happening as to a human player with negative economy? Or trade ends? AI goes to War?
I'll hopefully find the time to test it this week...
The idea for this came from the Aztecs in my current game, wanting no less than 120 GPT for Iron (although I have one of the smallest empires of all civs)... . Anyway, I made the trade (last Iron available IIRC), and then realized, they are now willing to pay up to 123 GPT for something - before I bought Iron they did have only few or even 0 GPT left to offer.
EDIT: Corrected 500 GPT equals 10'000 Cash