Whilst it's good that diplomacy has a resource dedicated to it, as I've mentioned in my previous post in the game mechanics thread, I fear that the immaterial nature of it in raw game mechanics terms ultimately cripples diplomacy against human opponents, though I'll try to keep such things to a minimum here and focus on the AI's trading in general.
Diplomatic capital needs to be able to buy things that give it real, tangible benefits to trade and other diplomatic pursuits. Want a neighbour to love you? Send over PR-guys (as marketting is inherently evil, death mages get a discount), who make AI and human players alike love you by giving them something. Want favourable trade deals with a neighbour? Send over sales guys who make trading better. The AI liking you more because you have some arbitrary resource, or "relationship value" that does nothing is a cop-out embraced by too many strategy games out there and doesn't lend itself to a decent AI anyway. AI players need to be self-interested, hoping to win, and, at least in the higher difficulty levels, every bit as ruthless, mean and unpleasant as a human player.
And I have to know.... why does the AI automatically offer an unfavourable deal? Why would the AI be arbitrarily unfriendly about it? Equal comparative value so long as you both need the materials being exchanged seems perfectly reasonable to assume as a baseline (though the expense of your contribution to a deal could be reduced by purchased game-mechanics effecting diplomats). 50 gold for 50 gold? Sure, why not. It wastes your (hopefully limitted) number of total trades per turn.
If you're trying for one-of-a-kind "dragon butt" resource however, then the AI might be reluctant to exchange at an equal price, just like a human player would, since they know you'll use that dragon butt for some kind of burning grenade equipment, and don't want you to benefit from that.
If a farming empire with 1000 food stockpiled was offered a slightly weaker deal for food (50g for 60g's worth), they might even agree to it. Why not? They have a thousand food, they're making more than they can use. Free gold, and you're indepted to them - and they might ask you a favour for it later, especially if they spy network told them your empire was starving without it.
Supply and demand are both relevant and important, and ideally - though I know working with AIs to do so is a pain - AIs should react as closely to a human player in this regard as possible.
The biggest capacity for abuse here is that a diplomacy heavy player could invariably profit on small, every day trades (with the right investment to put game mechanic effecting diplomats into place), offering 60g (but only spending 40g to do so) in exchange for 55g. Honestly, with a sensibly enforced maximum trades per turn, that sounds great. A trading/diplomacy nation should have this sort of option - heck, it should be their primary source of income, and one that makes everybody think they're getting the better deal out of the equation to do it (and two similarly diplomatic nations both thinking they got the better deal).
Who wouldn't want to play as a trading/diplomatic empire that's every bit as potent and wealthy as a magical or military one?