I play on Standard speed as Egypt in the scenario Scramble for Africa, and the double gold for caravansaries and harbors ability doesn't work as advertised. I don't have the screenshots or save games here but I did do some tests:
with <TradeBuildingModifier>100</TradeBuildingModifier>, Caravansaries gives extra 2 gold up to total of 4, Harbors gives 0 extra.
with <TradeBuildingModifier>200</TradeBuildingModifier>, Caravansaries gives extra 3 gold up to total of 6, Harbors gives 1 extra, up to total of 3.
with <TradeBuildingModifier>100</TradeBuildingModifier>, Caravansaries gives extra 2 gold up to total of 4, Harbors gives 0 extra.
with <TradeBuildingModifier>200</TradeBuildingModifier>, Caravansaries gives extra 3 gold up to total of 6, Harbors gives 1 extra, up to total of 3.
