Make the plate armor/plated charger/etc much more expensive and remove upkeep. Otherwise - to me personally - this mod is broken.
I think there is the mistake in your thoughts.
You know, balancing comes from balance. And balance always exists between two things.
What you are suggesting is a balance between you and the game, like in single player. You invest time and effort, and get rewarded by better equipment. For you personally this is nice, of course.
But you always have to balance out not only you and the game (the game = your gaming experience), but also you and your fellow players.
Let me just make an exeggarated example about what I mean.
Imagine you were a nerd without any real life, playing all the time. And imagine there would be an item, called the "Holy Hand Grenade of Antiochia". Once thrown, all enemies on the map die. This item costs 10.000.000 gold. So you would need to farm... I don't know how long. But after months ingame, you would finally get the item!
To you this seems fair. You worked hard for it, now you shall recieve your reward. Being fair is to be understood like being balanced in our case. It has the equal meaning.
But now imagine how all your opponents would feel like. Would they think it's fair they spawn and a second later they die (like white, bloodthirsty rabbits) because you throw the holy hand grenade? They would think it's unfair, which - as we stated - means it would be unbalanced. And yes, sure it is. Your opponents do not care about how long you had to grind for this item, because as soon as it's on the battlefield (and with your suggestion it would never leave it, too!) they have the problem with you and your item, and it doesn't matter if you got this item by grinding a year, hacking, winning a contest or giving head to chadz, there is a shift in power and thus in balance, no matter what enabled you to cause it.
The thing I want to tell you: you can't balance the game by things you basically have unlimited access to. Because gold (and experience) are basing on time, and - at least concerning the matter of balancing cRPG - you got infinite time, as it's only you who is limiting it. So there is actually no balancing at all.
But before you try to defend yourself: in my opinion upkeep is a bad way to balance things, too. You made the perfectly right sentence that items are to be balanced over their stats. Balancing their "availability" (by price, upkeep, drop chance or whatever) doesn't work. Because availability is always only a matter of time.
And yes, before some Einstein comes and says that RPGs are meant to have those time based differences in player power: of course! It's always a matter of personal (= the developer's) taste. I currently play Global Agenda, a SciFi 3rd-person-shooter-hybrid between Team Fortress 2 and World of Warcraft. You get loot by visiting instances in groups with different classes, gaining experience points, crafting items and so on, but you shoot yourself, there is no tab-targetting. Levels go from 1 to 50.
In GA a player with level 10 and good aiming has quite a chance against a player with lvl 50. The best epic rifle does only 3% more base damage + 12% additional damage, which is 15,36% more damage overall. So with better aiming you still got a chance against a lvl 50.
Now try to fight a lvl 50 WoW character with a lvl 10 character.
And in our case the developers have decided in the pre-upkeep era, that the time based differences between plate and new players are too big. So they lowered them. Which is basically okay, in my opinion. I just don't agree to both the developer's and your suggestion.
