[Sandman]

My understanding of unit costs is that a basic human soldier costs 10 gold, and has 10 in nearly all their stats. Each +1 to a stat adds +1 to the gold cost, whilst any national bonuses (like Marignon's +1 morale) are free. The Ulmish Guardian has +4 morale, +2 attack, +2 hitpoints and +2 strength over a normal Ulmish soldier, and consequently costs 20 gold. Some units exceed or fall short of this system, of course. 'Hoplite' units (Arco, Machaka and Pangaea) have excellent stats for their gold cost, in particular.

With this system in mind, the cost of the Medean infantry should be 15 gold, and the Persian infantry 16 gold (I'm ignoring their +1 precision) This can be reduced further if they are considered to have a national bonus to their morale, as their description suggests.