At least since 3.04, (might be earlier, I don't remember) a civilization will offer me a trade. Say they want a tech, some resources, and credits. The NPC offers a tech, and some resources. This is a balanced deal on the screen. If I remove the credits they want, the deal goes wildly in my favor say 80%. I can remove the resources they want. I can have the NPC give me more resources, another tech and some credits to reach a new balanced deal. So for a tech, I get two techs, several resources and credits. If my tech was that valuable at the end, why wasn't the offer at that rate at the beginning?