Regarding billmen, there are 4 types, from worst to best: Bill militia, billmen, heavy bill militia and billmen.
Their upkeep scales, but not significantly as you require higher level tech buildings to access better versions, and lower versions eventually phase out.
Think of the lesser billmen as peasants with a bit of training and a weapon. Higher billmen as standard heavy infantry.
Non-heavy bills have very little armor and tend to be effective in close quarters against cavalry - they are not meant to take a charge, but rather charge or pin down the cavalry. If you want a unit to take a charge, use spears or pikes - these consequently are slightly less good at killing cavalry in close combat and will fall more easily than bill type units against heavy infantry due to lacking the armor piercing attribute.
To make good use of light billmen, you want to be upgrading their armor using a blacksmith. Or use them mainly in flanking or urban battles. They will only be good against cavalry. Heavy billmen can deal with light-medium infantry and heavy cavalry. They still cannot take a charge, but will have better survivability and not fall to missiles as easily either.
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