This may sound strange, or at least I find it strange... the 9% interest rate happens because of the aggression level setting (which I found out last year in the Sid XOTM game, which I never finished or submitted). Had this game gotten played on "normal" aggression or some other level, you couldn't get 18 gold for 1 gpt... it's more like 14 gold for 1 gpt or something like that say on normal aggression. This has nothing to do with the level of the game.
Oops. Did some checking in DEBUG mode.
At less aggressive,
Osman Gracious -9 is willing to give 16 gold for 1 gpt.
Osman Gracious -8 will give 1 gpt for 15 gold. The negative numbers are the AI attitude.
Osman Gracious -8 will give a worker (I gave him one, then bought it back) for 119 gold.
Gandhi -11 Gracious will give 17 gold for 1 gpt, and requires 106 gold for a worker.
So, the numbers at least vary within a certain range.
I would be surprised though if I hadn't bought a worker from a least aggressive AI who was annoyed or worse with me though.
The AI attitude appears to be one factor, and aggression level changes default AI attitude. So, aggression level does have an effect, but it's not the direct cause of the minimum cost of a worker or the minimum exchange rate of gold for gold per turn.
Edit: At least aggressive:
Polite -2, 1 gpt for 18 gold.
Gracious -12, 1 gpt for 18 gold.
Furious 60 (couldn't seem to get any higher), 1 gpt for 18 gold.
Worker also cost 100 gold at furious 60.
So, it looks like the best exchange rate for the human player is constant at least aggression. But, for less aggressive the best exchange rate and worker cost can vary depending on AI attitude.
Thus, my previous comment looks correct for Moonsinger's game and other minimum/least aggression games, but when I've made claims about the cost for games with other aggression levels after just checking once, that's where I've been wrong.