Maybe Montezuma is an exception. We know that civilizations that want to achieve the Congress victory are willing to give double for Diplomatic Favor (well, it was the case, I need to check this). Either Montezuma now values the luxury ressources more, willing to trade 3 for 2 (because he gains 12 Amenities out of it), or is triggered to value them more because of his agenda (maybe "Fun-Loving" works the same). Unhappy civilizations are willing to pay more, and overly happy are willing to pay less.
After some testing:
It seems that I can trade my luxuries between at 4, 7 or 10 GPT but they are willing to sell them at 2 GPT. It seems civilization are willing to pay more if one of their city is not at 100/100 Loyalty, or if one of their city is displeased or unhappy. Maybe they would pay even more if most of their city are facing Unrest while loosing Loyalty.
I didn't have a save with an unhappy population that was loosing loyalty, so I cannot test if the AI will charge you more if you need the luxuries.
If a civilization likes you, the price of all thing you will buy and sell will raise. For example, if a Writing is 10 GPT, you can sell it at 12 GPT, or buy it at 12 GPT. You can sell at better rate but buy at worse rate. I guess they did that because if you could the Writing at 12 GPT and get it back at 10 GPT, it will leads to some exploit.
On the contrary, if the civilization dislike you, it will add a -5 GPT on any trade you ask. You can sell a Writing at 5 GPT (10 GPT - 5 GPT imbalance) at a civilization that denounced you, but 10 Writings can be sold at 95 GPT (10 × 10 GPT - 5 GPT imabalance).
If you need to buy something, it is the same + a flat additional cost. A Writing will cost me 15 GPT + 206 Gold (10 + 5 GPT trade imbalance + 206 flat gold), but 2 Writings will cost 25 GPT + 344 Gold (2 × 10 + 5 + 344*, 344 not being twice as 206 and I don't why). Basicly, you can still sell at the AI if you are enemies as long you sell all in one go for just a -5 GPT penalty, but you can't buy anything without paying a huge chunk of gold.
Prices may rise up depending on the target victory, or agenda. "Fun-loving" leaders will pay more for luxuries (not tested), while civilization like Canada or Sweden will pay a DF at 10 Gold instead of 5 (or 6 GPT the 10 DF instead of 3 GPT, tested).
Also, you can bankrupt a civilization easily. When you offer something and ask what they are willing to give you in exchange, they will give at least 1 Gold. If they do not have any flat Gold left: 1 GPT. So you could buy all their Gold with DF, then exchange 1 by 1 all your useless strategic ressource at 1GPT, then buying back at a ridiculous price and repeat the process until they do not have any Gold left.
For example, Canada has 230 GPT and 3200 Gold. I sell 300 DF for all his Gold, then sell 1 by 1 my Iron at 1 GPT. Then buy it back at 5 GPT, and repeat.
The AI leaders too polite and to do not want to upset you when you want to sell your silly Horses. They need to toughen up and ask you politely yet firmly to leave, instead to give you Gold. At least, they do not ask a 57 years GDP of your empire as Gold for 20 Niter anymore.
It is too bad that the trade screen does not work properly in other languages. "One Time Deals" becomes "Accords Ponc..." and "For 30 Turns" becomes "Pendant 30 To...". Also, I can't see how GPT they generate and have to manually checks. But for mass trading, the new trade screen seems better.