What I am really looking for is to say, "cities cost 300 g to found."
I think you want these lines in K_CityHubs.xml and F_CityHubs.xml :
<!--<GameModifier>
<ModType>ConstructionResourceCost</ModType>
<Attribute>Gold</Attribute>
<Value>-25</Value>
</GameModifier>-->
Uncomment and change the value to -300 :
<GameModifier>
<ModType>ConstructionResourceCost</ModType>
<Attribute>Gold</Attribute>
<Value>-300</Value>
</GameModifier>
IIUC creating a city automatically builds the "Outpost" improvement, to which you can attach improvement modifiers like build cost, capacity (10 population), etc.
