How about a "What do you want for X?" or a "What will you give me for X?" button?
As in, you select something from them you want, and rather then having to spend a long time trying to figure out what a fair trade is, they just suggest something. The same thing could work the other way, you select something you don't mind trading to them and they offer up something in return.
Obviously the AI would reserve the right to initially offer up a deal in their favor, but I think this would be more realistic. You could get something of a back and forth going. Say you selected a technology and they asked for 5000 bc. You could then offer 4000 bc and they might say no, but if you offered 4700 bc they might say yes.
Any thoughts?