Moderator Action: Moved to Ideas & Suggestions.
I don't think it'd really work, because trades generally fall into a number of defaults (that are determined by the strength of your friendship, not by whether it's really a good deal or not), and their fairness is dependent on whether you're going to declare war the following turn or not, or whether you actually need extra happiness for example (if you're buying a luxury), which varies from turn to turn and can't really be quantified at the beginning of the trade.