Armor has two stats: Damage Reduction (how many points of damage it takes off) and Penetration Resistance (how likely it is to turn a dangerous Penetration Attack like a sword or bullet into less dangerous Impact Damage).
Armor itself comes in two varieties: Undercoat (which covers your whole body) and Plate (which covers only portions of your body).
When a target with Plate is struck the amount the attack hit by may be reduced at the option of the attacker to hit around the Plate. If a Plate has a "Coverage" of 4 and an attack hit by 5 it can either go against the Plate + Undercoat as a hit-by-5 or it can ignore the Plate but is treated as a Hit By 1.
Remember: the more you hit by, the more damage you are likely to do.
When hitting a target with Plate Armor and having hit by enough to possibly get around the plate (if the coverage is 3 and you hit by 0-3 you automatically hit the Plate) you have to do the math and determine if you think it's better to keep your original hit or reduce it and ignore the plate. If the plate is enough to always stop your attack then you'd want to reduce the Hit-By number always. If the Plate is wimpy, though, you may want to hit through it.
Okay ... So What?
Well, that makes "costing Plate" pretty difficult. Here's why:
- Plate Cost is based on coverage. If the Plate has a coverage of 8 then it's basically just regular armor. So few attacks will hit by 9+ that the Plate is almost always in play.
- Plate Value is based on AGI Bonus. If we wanted to be really technical about it we'd base Plate cost on the wearer's AGI because if you have an AGI bonus of 3 and an Plate with Coverage 4 the attacker needs to hit by 7+ before they have a chance of choosing to hit around your Plate. If your AGI is 10, though, that number is 4+. Fortunately even we are not that anal (we also don't, as a rule, factor for regular character stuff).
- Above a certain level Plate simply acts like more AGI bonus. If my Plate is so high that it'll stop any attack then it just acts like "being missed" (that is: if the attacker does not hit by enough to hit-around-the-plate then their attack hits the plate and does 0 damage ... as though it had missed).
A Note About Infinite Armor
There is a related argument that an "infinite amount of armor" should have some finite cost in terms of the game. The argument goes like this: In a given game I can reasonably expect only a limited range of attacks coming at me (as a general matter of course). If I am able to buy enough Armor to ignore these attacks then any armor purchased above that number is deeply into diminishing returns. A 'correct' cost for this would have the cost-per-point-of-Armor approach 0 as I keep spending more.
Another way of saying this is this: if the game tends to keep attacks 'balanced against the characters' then if a 24 AP character spends all his AP on Armor it won't matter much if he gets 24 Armor (what the standard cost would give him) or 240 Armor ... if the average attack is just as useless against each one.
Now, this argument falls apart pretty quickly in 'real games' where if a character has 24 Armor they might stand up to a rifle -- but if they have 240 Armor they will happily go up against a heavy machine gun. The fact is that it does make a difference but, for a lot of the play, not much of one.
However ... When You Get To Plate ...
The argument suddenly makes much more sense. If I am attacking a target with enough Plate that I want to hit around it always (which is not that much Plate, really) then whether they have 24 Plate or 240 Plate is a lot less relevant: when I am only hurting them when I hit around their extra armor it doesn't much matter how much extra armor they have.
That's What Our Simulator Says
At 16 AP 1-3 points of Plate Armor costs just slightly less than Undercoat ("normal armor"). Above that, however, each extra point of Plate gives you almost nothing. A character with 5 points of Coverage 3 Plate and a character with 8 points of Coverage 3 Plate perform almost identically against the 16 AP Herd.
Unless I've done the math wrong, at the 32 AP Level 8pts of Plate costs 6 AP (instead of the 8 AP it would cost if it were Undercoat). But 12 points of Plate just tests as costing 7 AP. And 100 points of Plate? It's valued at only 8 AP.
What Do We Do?
Well, the cost of Plate certainly is a function of Total AP cost (TAP) and, weirdly, it maxes out: I don't think you can literally buy 100pts of Plate armor if you are 32 AP--or, if you can, whatever you pay almost certainly isn't worth it.
Here's what I think: if you want small amounts of plate (where the costs are incremental) you have to pay for that. But once you reach the limit (about 1/4th of your AP, looking at it--but I'm not sure yet) then you get "maximal Plate" for your level (we need to determine what that its--but it's not 100pts of armor. It's probably something like 1.5x the AP cost).
Finally you can probably buy MORE plate if you really want to. Maybe 4 Armor for 1 AP or something like that--something that's statistically a bad deal but still fairly tasty.