Game Version: Gathering Storm, September 2019 update (v.18.104.22.1681)
Spreadsheet can be downloaded from the Discussion tab, or by clicking here.
This reference calculates some of the core stats of military units, needed to understand relative combat strengths and the relationship between str and production costs for the different classes.
The first table shows all non-unique military units and how they fare against each other based purely on base strength and class advantage bonuses. The values listed in the table are the resulting Str difference (d.Str) when the unit listed on the left is attacking the unit listed in the top row.
d.Str is ultimately what decides the result of combat according to the researched combat formula. From my testing, the combat results fall broadly under these categories:
d.Str = 0 ~ Stalemate
d.Str < 5 ~ Minor Victory / Defeat
d.Str >= 5 ~ Major Victory / Defeat
To halve an enemies HP from 100/100 to 50/100 requires, roughly, d.Str = 12 or so. To one-shot an enemy requires, roughly, d.Str = 25.
The next graph shows melee strength vs production cost for the different types of units. For ranged units, the Ranged Str is used here, not Melee Str. Similarly for units that use bombard type damage, the Bombard Str is used. Vertical lines separate the eras that units belong to (Ancient, Classical, Medieval, Renaissance, Industrial, Modern, Atomic, Information). Note: last point for Anti-Cav belongs to Modern AT and while it has a production cost typical of the Atomic Era, is actually an Information Era unit.
Finally, the same information displayed in terms of str/cost (think of it as cost-efficiency). Note: last point for Anti-Cav belongs to Modern AT and while it has a production cost typical of the Atomic Era, is actually an Information Era unit.
Perhaps the most important thing to note here is by how much the cost-efficiency drops when producing later-era units. This highlights the importance of trying to build your army as early as possible, keeping it alive, and upgrading it with gold instead of producing units later in the game. (Assuming you wish to build a strong army). Yes, this cost-efficiency is displaced by the gold used to upgrade, but using gold is much better than production, plus you get to keep promotions which can be a massive advantage.
Ignoring gold costs, a warrior built with 0.5 cost-efficiency (and upgraded to Mechanized Infantry) gives you 3.5x the "bang-for-your-buck" that a Mechanized Infantry will give you, produced in the Information era at only 0.14 cost-efficiency.
Hope this information is useful for analyzing combat and designing mods. Enjoy.
Units Relative Strength and Costs Reference
Who beats who and for what cost