it seems that you only need gold to upgrade a unit form 3 to 4+ members.
This can be abused, ie if i only have 3 horses, but i can build a unit of 5, I just build a unit of 3 , then it costs me just gold to upgrade.
I would suggest that to upgrade the numbers of a unit it should cost a price & all the extra resources you would need to make it.
it works this way with weapons & armour. So let just add it to the unit size as well.
