I think a single archer could be killed by a single Ninja. The real question is where the equilibrium lies with Multiple Archers vs Multiple Ninjas (if 10 archers vs 10 ninjas, who has the advantage)
I also agree that One archer should be able to kill several units if they have an inefficient way to get past the walls (ladders, battering ram) ... however one archer vs two equivalent units + catapult would be interesting "skirmish".
Is it possible to have decimals for unit-upkeep?
Frogboy's post made me think that a single soldier unit should probably cost 0.1 gold in upkeep, and a party could cost 0.3 (25% discount) gold, and a company could cost 0.73 gold. (27% discount)
In this way, a company is indeed cheaper than ten individual units, as advertised in the tech tool-tip.
a unit of 50 soldiers could cost 3.5 gold per turn (30% discount), and a unit of 100 soldiers could cost 6.5 gold per turn. (35% discount)
a unit of 250 soldiers could cost 15 gold per turn (40% discount), and finally a legion could cost 50 gold per turn (50% discount)
And finally, (for soldier upkeep only) maintenance costs would be rounded down to the lower whole number at each juncture. Thereby, if you only have two individual units and one company, "expected" payment is 0.93 gold, however rounding down, this means no maintenance (or free units).
So that when you have enough units to go over 1 gold per turn (10 soldiers, 3 parties, or 2 companies). Personally I think this is the best way to handle unit maintanence, given the example from Frogboy's post.