One way to ensure that you get the maximum bang for your buck when trading resources for cash is to first setup the initial deal for whatever amount of gpt. Then monitor every so often how much gold they have available under the F4 resources screen, and when they have some extra gold, you can cancel your deal (after 10 turns of the original) and then setup the new deal asking for that extra gpt. Unless something has happened to affect them liking you, they should take the new deal, and you are sitting happy with a few extra gpt. If you do this to a number of trading partners, you can continuosly build your gpt while slowly eating away at those of your opponents.