You can also just change the trading difficulty from Average to Very Easy, which usually does the trick.
Main Menu > Settings > Trading Difficulty
I just tried this and the exact same trade went from "That's a very bad deal for us" to "That's a rather fair offer, barely." It's technically cheating, I guess, but sometimes the game cheats you, too.
Just remember to change the setting back later to make the game fairer to the other teams.