[General Tacticus]

If you look at Pythium's two themes, Hydra cost 250 gold in normal theme and 300 gold in serpent cult, where they become sacred. Likewise, Hydra hatchlings go from 50 to 60. On the other hand, Serpent Cataphract stay at 60 gold.

I think part of a unit cost is dependant on nation, with some nations/themes having deliberately lower cost for certain units (like Ulm paying 10 gold for his heavy infantry, where similar units are 15-25 gold for other nations). Still, sacred costing +20% gold seems about right to me...