hi Abaxial ,
the 'quality' of trade deals is for the most part tied to diplomacy. If you have a bad relationship the AI they will propose unfavourable trade deals. However, if you can maintain a good relationship - then the deals get much better and AI will normally offer between 7-11gpt per luxury (rarely slightly less, and often, slightly more), even more gold for strategic resources and around 19-23gpt for each work of writing.
Notable exceptions are, for example, when the AI is pursuing a particular victory condition - then, if they focus on religion, you can sometimes sell a relic for up to 40-50gpt. Similarly, great works of music can also attract very good trade deals.
Now, sometimes it is also possible to get great deals from AI that do not like you - if for example they have recently lost a number of units in a war (not against you) they may be willing to pay even up to 20gpt for a luxury (to fight off war weariness).