Interesting analysis - I did notice the fact that there were many more uber-units that were almost exclusively Gold-dependent.
So my assumption was that Gold expense was to simulate what people call "human capital", capital that come from resources that do not directly contribute to their construction. Hence, mages, mercs and hydras don't require much Resource because the time and expertise used in their raising are not directly applied to their construction.
I understand that Resource does travel to a limited extent through the use of Forts, but it is nowhere near as universally applicable as Gold, which sits in a single pool. Hell, even Gems, which sit in a single pool, require a Lab to produce effects in a certain Province.
While the Resource cost of specialist units -could- be adjusted upward to compensate for this, I believe this would reduce the distinction between gear-based units and skill-based units. Unfortunately, I can't think of anything better ATM
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