I wonder about that too. Last night in my game, I watch my gold income go from +200 to +130 and then back up again - all from trades coming in. Sometimes I see 7 trades coming in and other times, none. I pretty much do the same thing as I re-evaluate where to send a ship when it comes time to renew. One of my primary decision points is whether to allow or not allow the two cultural runaways (Siam and Iroquois) bonuses for trade partners - comes a time that I would sacrifice 10 gpt and a science or two just to slow them down a bit.