The money gained from performing a diplomatic mission is 350 + (50 * <era number>) where era number is just a serial number starting with 0 for the ancient era. In addition game pacing also affect this number: quick games modify this value by 0.67, epic by 1.5 and marathon by 3; or in other words:
Gold in Gold in Gold in Gold in
Era Quick Standard Epic Marathon
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Ancient 234.5 350 525 1050
Classical 268 400 600 1200
Medieval 301.5 450 675 1350
Renaissance 335 500 750 1500
Industrial 368.5 550 825 1650
Modern 402 600 900 1800
Future 435.5 650 975 1950
I know of nothing else which can affect the gold amount.
Now, if I have a strong economy in the late game I typically gain another 100-200 gold per turn when in a golden age. That means that the only way in which a golden age is preferable to a diplomatic mission is when it lasts at least 3 to 6 turns. But usually by the time I get to the late game my golden age duration from great people is really low, and if you take into consideration that the influence gained is worth a few hundreds in itself, you'll see that a golden age looks less and less profitable in comparison. All this is especially true if you play in longer game speeds, which translate to higher gold.
Of course, golden age also nets production, so it's still a dilemma; but I do believe merchants are worth it from the financial aspect.
