Ok, i'm lvl9 skill on crafting plate covered shield, 45% discount. So what ? I won't equip 100 men with a 90 gold shield, I just buy them some "4 coins shield", quite enough against arrows before close combat.
Look : if I need 20 shields, I could craft those plate shield in 20 hours. Total : 20x90= 1800 gold +20 (city cost)
20 hours crafting goods , I gain 240 gold - 20 (city fee) = 220 gold. I buy 20 4 gold shields: 220-80= +140
First case cost me 1800 ( almost 200 hours of craft working to build+making money)
Second case : I have +140 gold, so I can travel to recruit more soldiers etc.
In 150 hours, you just have 20 big shields, I have a fully light equiped army of 100 guys...