I have just done a test (sort of) and found something strange with the AI behavior in diplomacy. Obviously the AI never was flawless when it came to diplomacy, but this interested me. First screen-shot: Doing a trade with Elizabeth. Second Screen-shot: I add Ivory and don't charge England anything more and strangely, they don't like the deal Third screen-shot: I get rid of ivory and make the deal in screenshot 1. But now, I offer to trade Ivory with England, and they will pay more for Ivory by itself rather than Ivory and contact with the celts. I also tested it without selling contact with the celts and then offereing ivory, but the same results happen. Now this is just strange behavior. I got a lot more gold per turn selling Ivory and contact with the Celts separately, than I did with them together. So next time, try selling things separately and you might get a better deal.