Since testing and feeling out the protections of each tier is the motivation for the even HP at present, this is all just fodder really.
All the same, it is worth noting that the amount of resources a builder has expended on his refuge are not limited to only the walls, foundations, nor other building elements, but also the LOOT those elements are intended to safeguard. I think it is indeed reasonable to be required to dump 4 charges into an armor-plated barrier before it will give way, regardless of the resource cost of said armor. If a group of raiders stands to walk away with 5 (or more) inventories full of armor, guns, resources, etc., then they should have had to spend an inventory or two worth of charges to get there.
To adequately do justice to a discussion about balance, all variables need be considered.
My two (obsolete Canadian) pennies.