I've never understood why the AI gives you discounts when you partially research a tech (in trade, that is). It makes the game easier for you, but doesn't make much sense (or rather, it seems kinda silly how they know how far you've researched even with EP). What I also don't like is how the AI places almost 0 value on monopoly techs in actual trade, at least compared to what I remember in civ3. In civ4, you can trade one new tech, say Philosophy, to each AI and they would all pay you a lot (if they hadn't been researching it themselves). Whereas the older system dramatically scaled down how much they would pay if a couple of their friends knew the tech, so this couldn't work so well (but I think they paid more if they were the first ones offered the deal). For that matter I also think the AI "hiding" it's gold is broken as well - it should just not be willing to trade it. (And of course I could rant on a lot more, like about the AI giving peaceful vassals everything... but this isn't relevant to your question, on the espionage side your/their research into something has no effect).