A quick answer for Poland (perhaps I’d elaborate on it in the future).
I think the folowing units are of peasant quality, their extensive use as garrisons for public order may be unhistorical and seen as a (minor) exploit, and, therefore, they should have “is_peasant” attribute:
Hunters, Woodsmen, Lithuanian Skirmishers, Balkan Archers, Lithuanian Archers, Lettish Crossbowmen, Mercenary Crossbowmen, EE_Crossbow_Militia, Slav Mercenaries.
(of course, the units which have “is_peasant” should continue having it).
I hesitate about two mercenary units: Axemen (available in Russia) and Macemen (available in Germany).
The units which definitely should stay as non-peasant since they’re more-or-less professional: Eastern Spearmen, Mercenary Spearmen, Prussian Spearmen.
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The formula to count the costs-effectiveness for garrisoning purposes:
= [ ( upkeep_price / number_of_soldiers ) * (is_peasant) ] + [ ( recruitment_price / number_of_soldiers ) * 0,02 ]
Where: is_peasant is 2 for the units having this attribute, but 1 for the others.
The lower the index, the better.
Remember that the mercenary units usually have a high recruitment price and lower availability, so when one takes into account taking loses, they become more expensive. On the other hand, they have usually lower upkeep, so they suit the garrisoning purposes better. There’s also the question of mobilization of funds at the beginning (example of similar units: you pay 2800 on turn 1 for Mercenary Spearmen, but only 450 for Eastern Spearmen at your castle, but then the upkeep per turn is 340 vs. 390)
With my changes the results are:
Peasant Infantry 1,7
Spear Militia 3,3
Slav Mercenaries 2,3
Eastern Spearmen 2,7
Mercenary Spearmen 2,6
Prussian Spearmen 2,7
Mercenary Crossbowmen 3,2
Lettish Crossbowmen 2,9
EE Crossbow Militia 4,2
Balkan Archers 1,8
Levy Archers 2,2
Lithuanian Archers 3,4
Hunters 3,8
Woodsmen 3,6
Lithuanian Axemen 3,8
Lithuanian Skirmishers 5,2
I also think some prices of the units should be adjusted as far as the division between recruitment vs. upkeep costs is concerned. It is both for gameplay purposes, but also for historical common-sense (eg. why Lettish Crossbowmen have such high recruitment price, while other crossobow much lower? Wasn’t it the crossbow easy to learn and comparatively cheap? On the other hand – why EE_Crossbow_Militia has the upkeep of 255, which the Lettish of 156?)
But it’s a completely different topic.
And I would like to underline: the SSHIP is way, way better in setting prices for the units than any other mod. It’s really the advantage of the SSHIP. What I have in mind here (both garrison issues and prices) are very minor issues, just tinkering with a very good mod.