The math is 33 gold per gpt when the AI is paying with gpt. The 30 vs. 33 has nothing to do with AI displeasure.
As odd as it may seem, an AI with a DOF will pay 240 gold up front, but will only pay 218 gold (7 gpt x 30 turns + 8 gold) for a predominately gpt trade. All I can figure is that the AI is programmed to be super-conservative on gpt trades (perhaps to minimize the risk that its income will tank at some point during the 30 turns and it may have to disband units).
Of course if you want to pay for a luxury with gpt, you have to pay 9 gpt (total 270 gold over 30 turns).
Equally screwy is that the AI will accept 5 strategics (total 225 gold value) for one luxury. But if you were to sell the strategics to the AI one-by-one, you could get 10 gpt. You could then buy one luxury for 9 gpt and pocket the 1 gpt difference. Exploit, yes, but hardly a quick way to get rich.