@ATEX Just finished my first game since this post - Deity domination as Rome w/SS and no other modes. Notably Georgia was in the game and a distant neighbor. While she didn't pay an unusual amount, 5g per 1 diplo, she would purchase the diplo immediately once meeting her in the ancient era. If "flipping" diplo around T60 is banned or just not your thing, Tamar is a good early meet to start banking off your early diplo.
Nothing out of the ordinary with selling luxuries. Korea offered me 11gpt for luxes consistently through the game until I betrayed and attacked them. I didn't get a chance to test Alex, as he declared on me sub T30 and my Legion counter push ended him so were never friends. I'm pretty sure there's nothing special about him valuing luxes or diplo from past experience. I met Gaul and Scythia super late since they were in remote locations on a peninsula and island respectively. But again, I'm pretty sure they're nothing special in trades either.
Spain was the other Civ in the game, who didn't seem too interested in diplo or luxes in my game. On an unrelated note, I was able to declare a protectorate war against him, triggering the inspiration for Nationalism. So mark me down for exactly 2 protectorate wars in almost 3000 hours.
Sorry the info is light, but I confirm Georgia pays early and a decent amount. Not the ~9 per you can get from Kristina but also not low balled either. I'll return to this thread in the future as I continue my quest.