Currently, there is no way to check/change the global upgrade cost of creatures.
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GetGlobalCreatureUpgradeCost(value)
Returns the global value utilised to calculate the upgrade cost of creatures.
In original Heroes 2, this global value is "2". How is that calculated?
The upgrade cost of creature is calculated based on the price difference between the price of the base creature and the price of the upgraded creature.
This difference is multiplied by 2 to get the upgrade cost of creature.
This is applied for the cost in gold and the cost in resources.
So, the upgrade cost of creatures is 2 times the price difference for the gold and for the resources.
SetGlobalCreatureUpgradeCost(value)
Sets the global value utilised to calculate the upgrade cost of creatures.
Example of use:
SetGlobalCreatureUpgradeCost(1);
With this setting, the upgrade cost of creatures is equal to 1 time the price difference between the cost of the base creature and the cost of the upgraded creature.
Therefore, only the difference in price.