pricing

It's 1120 AD, turn 173, and I'm desperate to buy niter. My ally Arabia has plenty. If I ask them what they want for 1, they say 7 gold. If I ask for 2, they say 6 gold + 1 gpt. If I ask for 3, they say 3 gpt. If I ask for 4, they say 5 gpt + 2 gold. If I ask for 6, they say 8 gpt. However, I...