5g no matter what the era, no inflation.
Other civs will never pay more, and their luxury's are always 8g no more, no less.
I play on Marathon.
Thanks!
Just like in Vanilla and BNW, AI's value their GPT at more than your GPT.
Assuming you are on good relations with them, your 1 GPT is worth 30 of their gold on normal (x3 for 90 on marathon). This is different than Vanilla and BNW, which was 22 gold, or 45 gold for 2 GPT.
Luxuries are worth 240 gold on standard (720 on marathon), so it works out to be 8 GPT for both game lengths (the x3 modifier cancels out).
Their 1 GPT is worth 42 of your gold on standard (x3 for 126 on marathon). This is different than Vanilla/G&K, which was 31 of your gold. 5 GPT is thus 210 (of 630) gold, which is below their valuation of 240/720. If you are Friends with them, you can get the change in lump sum gold but that is not possible if you do not have a DoF. 6 GPT would be worth 252 / 756, which is more than they value the luxury. That is why they won't accept them.
Note that on STANDARD speed, they accept a little bit of rounding. In G&K, if you want to do a research agreement and are one era ahead, you needed to provide exactly 100 gold (so Open Borders + 1 strategic resource + 5 gold). But I have found that in BNW, I can sometimes get away without providing the 5 gold in change, so just Open Borders + 1 strategic. Most notably, you also get this with Accept Embassy and Open Borders. The face value of these are 25 and 50 GP, but they will accept 1 GPT (from either side).
One clever trick you can do: If you have a DoF, do NOT accept an Open Borders for Open Borders trade for them. Instead, you should sell them your Open Borders for 50 lump sum gold. And then buy their Open Borders for 1 GPT. Since they value their 1 GPT at 42 gold, you make a tiny profit off of this! However this does not work for Marathon because the Embassy and Open Borders cost do not scale.