The basic recruiment cost is detemermind by a few thing:
first we have a base cost for levies, proffesional warriors, elite units,... for exampl 0.8..
every piece of armour (shield, helmet, greaves,...) has a set cost variable, for example a cuiras costs 0.7 (this is different for every piece of equipment)
Than the corst of a horse/chariot is added (also differs from types of horses, an unarmoured horse for example costs 1)
Then every weapon has it's own cost. An overhand spear costs 0.05.
Certain special bonusses like a high skill level can also have an additional cost.
All these are counted together then multiplied by the number of men and a standard value (let's call it 'k'). Certain factions get bonusses on specific units, for example the sarmatians get a 30% discount on horses.
Now as you can see the cuiras has a cost of 0.7 this amounted with the helmet, greaves,... multiplied by the number of men multiplied by k > 0.05 multiplied by the number of men, multiplied by k.
Which is realitic too. Improving a bunch of spears is rather cheap compared to improving a whole set of (heavy and high quality in this case) armour.
hope this helps.

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