I can only give a CS gold in double increments - 500g/1000g/2000g - as opposed to the standard 250/500/1000. These give the appropriate amount of influence - 75/155/315 - on turn 57.

Following up, I am about halfway into the game and find that paying 1500g sometimes leaves me as a Friend. This is because the AI has plenty of gold at the higher levels, and just pays more without noticing.

Thanks for pointing this out, I found and fixed the problem. I was experimenting with some things and forgot to remove the changes.