A detailed look at Penetration and Resistances, and the disparity between armor and magic resist

I'm sure I'm not the first person to comment on this topic, but the changes in itemization for MR vs Armor feel very lopsided and cumbersome. I'll explain.

Armor pros: Incremental buy steps (cloth armor to chain vest) allows for different tiers of armor bought with multiple price points. Price points are easy to buy (300, 720) for first 2 tiers of armor items. Can buy an armor item at level 1, which can then be upgraded in mid-late game. Upgrade path makes sense and has clarity. Several good late game options with varying utility. Large armor range for available items (15 - 100 armor available). Can stack massive armor in a single item slot (Thornmail, Frozen Heart at 100 armor each) Almost all late game items are useful with their active/passive abilities in some capacity.

Armor cons: Randuin's Omen is arguably the best in slot for value of armor/tanky stats for it's cost and item build path. (700 HP, 70 armor, bite sized prices to build this item), which does not encourage item diversity.

MR Pros: Only 1 MR item required for each late game item leaves less chance of making an item purchase mistake.

MR Cons: No multiple tiers of base MR items (which also means no upgrade path of base MR items). No late game items use multiple instances of the single base MR item (one null-magic mantle in each item build) Low MR stats for mid game and late game items with a very small range (25 - 55 MR available). Cannot start with an MR item at level 1 unless you go deep utility mastery tree, which leaves no room for any other items or consumables. Very difficult and expensive to stack MR. Getting 100 MR without filling up all your item slots with null-magic mantles would require 2 late-game items in the range of 6-7k gold. Magic penetration is more efficient than Armor penetration because of the cost to get MR vs the cost to get Armor, plus the base penetration stats are higher on Magic Pen items vs Armor Pen items.

Brutalizer - 1337g for 10 Armor Pen Ghostblade - 2700g for 20 Armor Pen Black Cleaver - 3000g for 10 Armor Pen and up to 25% Armor Reduction Last Whisper - 2300g for 35% ignore Armor before Armor Pen

Haunting Guise - 1485g for 15 Magic Pen Sorcerer's Shoes - 1100g for 15 Magic Pen Abyssal Scepter - 2400g for 20 Magic Reduction AoE (arguably Pen) Void staff - 2295g for 35% ignore Magic Resist before Magic Pen

30 Armor Pen (Brutalizer + Ghost Blade) = 4037g (134.567g per point of Armor Pen) 30 Magic Pen (Haunting Guise + Sorcerer's Shoes) = 2585g (81.167g per point of Magic Pen)

(Yes I am aware that the Armor Pen items also have higher damage stats than their Magic Pen Counterparts)

Cost of Armor - 55 armor for 1050g in 2 item slots OR 45 armor for 1050g in 1 item slot Cost of Magic Resist - 50 MR for 1000g in 2 item slots. No single item slot options available.

Alternatively, to directly counteract the penetration stats you could also consider this: Cost to directly counteract Armor Pen: 600g for 2 cloth armor (30 armor total) Cost to directly counteract Magic Pen: 1000g for 2 null-magic mantles (50 MR total), OR 1200g for Spectre's Cowl (35 MR) - there is no low-cost way to 1:1 counteract the penetration stat.

Conclusion: While the cost of armor and magic resist at base levels are relatively comparable, Magic Penetration is much easier to buy. Armor penetration is 65.7% more expensive than magic penetration based on a cost of 134.567 gold per point of armor penetration compared to 81.167 gold per point of magic penetration. Add to this less base items to buy magic resist in different intervals (another disparity of 66.7% cost at the lowest level to match or exceed the respective penetration stats - 600g for armor compared to 1000g for magic resist) and what we have is that magic based abilities are much harder to itemize against than physical damage based abilities when the opponent stacks penetration items.

Ways this can be fixed:

1. Increase the diversity of magic resist items at the base level to match closely with armor items. Going back to 2 base items will allow for fixing the defensive price point. If the null-magic mantle were to also be put as an upgrade to lets say the 'negatron cloak' that was recently removed, a consistency could be established between the armor and magic resist stats.

2. Increase the overall cost to acquire magic penetration stats to be in line with armor penetration. This can be done by either decreasing the amount of magic penetration on an item (for instance reducing Haunting Guise magic penetration from 15 to 10), increasing the cost of the item (and thereby increasing the stats of that item to compensate) or some combination of both (keeping an item's cost the same while adjusting the stats accordingly).

3. Increase the range of the magic resist stat on items. From a current range of 25-55, lowering the bottom end and increasing the top end can give greater diversity of item choices and build paths.

TL;DR This is a case argument that analyzes a cost efficiency ratio disparity between armor penetration and magic penetration. The numbers come out to the cost of armor penetration is 65.7% more expensive than magic penetration and that magic resist can be up to 66.7% more expensive to counteract magic penetration from items, with fewer good high MR options.