This is what I've been thinking about with all the ease of exploiting the AI when it comes to diplomacy. Why is the AI so easy to exploit? Here is what I would change...
First off lets say Custom Faction A has a high priority on Research and a low priority on Culture, with everything else in between....
I would make it so that this AI is going to pay more in either credits or other items for anything that will help them accomplish their goal to obtain research goods. This means They will place high priority on Research Treaties, any research tech that helps improve research, and the trade resources that help boost technology. This also means that I would have them put a lower value on anything culture, such as Culture treaty, culture trade resources and, techs (although they still want general techs because that's their goal.
The next question I'd ask if I was the AI, is how does this help me or how does this help the other player making the trade? For instance I'm Human Player offering Custom Faction A a Free Trade Agreement (which provides a 25% boost to economy for the receiving player only.) This means as a human player I'm looking for something of value for this. As Custom Faction if my economy is 100 credits per week, this means that I'm getting a boost to my income of 25 credits per week. I believe these agreements typically last for 40 turns, which means if my economy holds steady I will gain 1000 extra credits over 40 turns. If I was the AI I would not offer more than 1000 credits for this agreement because it's going to take me that time to get that back and Human player is going to get it now. Now I understand that Custom Faction A's economy most likely will grow over those 40 turns and get more than that but, that's to their advantage and that's the cost to the Human player for asking for the credits up front.
Now cross trading, Human player is offering Custom Faction A, 3 production trade resources that boosts my manufacturing by 5% each. As an AI I look at this and see that overall this will increase my manufacturing by 15% overall, now how to give that a value? Well let's say I currently can produce 1000 manufacturing per turn, I now would produce 1150 per turn. Let's say for arguments sake that 1 manufacturing point per turn is worth 1 credit. This agreement is also for 40 turns. So as Custom Faction A I would see a total value of 4600 over the course of the next 40 turns for this trade resource. Now again as buildings complete and techs get researched this may expand, however, once again that's what you get for obtaining this now versus letting yourself grow over time.
One could also apply similar ideas and formulas to various techs being traded ect... Now at the beginning of the game these numbers are small as you see above, however, late game this can be huge. See below..
I'm a Human player with 150,000 manufacturing points per turn and I'm attempting to purchase 1 manufacturing trade resource from Custom Faction A. This will boost my manufacturing by 5% or 7,500 per turn. Once again this is a 40 year agreement. As Custom Faction AI sees this is a value to you of 300,000 credits. Right now in the current system I can obtain this from the AI for around 1,500 credits (and if I want more than 1 in that trade I can have as many as the AI has to offer in the current system for that one trade.) No More AI cheese there...
That would work the opposite though as well using that same trade, I offer 1 economic trade resource in return that provides 5% increase to economy. Let's say late game their economy is 50,000 credits per turn. This means That the AI gains 2,500 credits per turn, meaning now the same offer still has a 5,000 credit per turn to the AI, so as a human I would still need to balance this out. Still less AI cheese.
Lastly Let's say human player A wants to obtain Planet A from Custom Faction A. Said planet is a class 4 planet, with upgrades potential to class 10. current production on planet for economy, culture, production and, research is 100 points per turn combined. This is a forever trade which what is forever? Well this is where I would have the game look at equivalent games first at player history, if none exists than average in community, if no Internet access than set number in game. So let's say for an Insane map 1000 turns. This means I will look at this planet as 100 points over 4 tiles = 250 points over 10 tiles. Game lasts 1000 turns, we are at turn 100 currently, This means 900 more turns to go, I'm going to request at minimum 225,000 or value for said planet. And with that being said it's the human player's dumb stupidity for trading for a planet in the middle of my culture where it will be flipped with in the next 20 turns right back to Custom Faction A. On the other hand if Custom Faction A is being offered the same planet, it will make the culture flipping calculation. Will this planet be culture flipped if I obtain it? If the answer is yes than no trade no matter how much value is put into it. However, if right next to my border and will not be flipped than why not?
Okay now that I've lost many people trying to understand my thoughts on how the diplomacy process should work anyone have any other thoughts on this?