From 1974 to 1985 there was no "full plate" armor in D&D. "Plate mail" only gave an AC of 3. According to the conversion document, this was equivalent of AC 16 in 5e, what you get from breastplate plus the allowable Dex bonus. Personally, since I like to emulate an old school style and feature a medieval level of technology in my campaigns, the comparative rarity of higher qualities of armor suits me just fine. In fact, I use half plate and plate to represent a higher level of technology than is available to the average person, and splint for an exotic technology that is just as rare.
Full Plate Armor (AC 1)
Description: Full plate armor is the best armor a warrior can buy, both in appearance and protection. The perfectly-fitted interlocking plates are specially angled to deflect arrows and blows, and the entire suit is carefully adorned with rich engraving and embossed detail.
Campaign Use: Suits of full plate armor are as rare as powerful magical items in most fantasy campaigns. Magical sets of full plate are artifacts to be treasured and hidden away, the objects of glorious quests.
In most campaigns, the number of sets of full plate armor can be counted as easily as the numbers of crown knights who owe their allegiance to the king. In many kingdoms, it is a crime to possess a set of full plate armor without royal permission, as a wise king keeps any armorer capable of such craftsmanship at his beck and call.
Full plate armor is one of the greatest gifts a great lord can bestow upon his followers. It is a prize as coveted for the status it confers as its monetary value. A suit of full plate armor will often be a gift presented to great knights upon great service to the realm, or as an incentive to attract a knight errant of unquestioned prowess to the king's private circle.
In addition, full plate armor is the most technologically advanced armor available in the later medieval and high chivalry settings. The special touches and custom enhancements added by the few living master armorers are what give full plate armor its increased armor class rating over the more traditional forms of field plate. At prices that start at 4,000 gold pieces for a simple, unadorned suit, full plate armor represents the crowning achievement of the armorer's ultimate goal--to forge for man a new skin of steel, as flexible as his own, but as invulnerable as anything in the land.
and
Field Plate Armor (AC 2)
Description: Field plate is actually a more commonly used form of full plate armor (described later). It consists of shaped and fitted metal plates riveted and interlocked to cover the entire body. Like plate mail, a set of field plate usually includes gauntlets, boots, and a visored helmet (see Helms ). A thick layer of padding must be worn under the armor.
Campaign Use: This armor is rarely used, except by noble knights on a military campaign. In theory, the bulk of a set of field plate armor is so evenly distributed over the whole body that the encumbrance rating of field plate compares quite favorably with that of plate mail and banded mail. In practice, the increased protection is paid for with reduced mobility and increased fatigue.
Each suit of this extremely rare and expensive armor is custom-made and fitted for its prospective wearer. Only a master armorer can create field or full plate armor, and only a master armorer can re-size captured pieces of a suit for a new owner. The new owner must be of at least a similar size and build as the previous owner, or the effort required to modify the piece in question exceeds the expense and effort necessary in forging an entire new set.
Source:
http://www.comingstorm.it/DA2/info/Books/AEG/AEG00000.HTM it's not "the" manual because I have no pdf version of ad&d tomes, but it reflects what's inside of it
This is what I meant about plate suits being costly and rare.
In 5th edition they drastically simplified suits of armour and shields, but somehow they wanted to keep plated suits being a "luxury" item rather than something common (in facts all the "plate" armours cost more than 100gp, mind you). Also this whole charade interferes with the fact that a +1 suit of armour is meant to cost 500gp, thus a full plate would cost 1/3 its nominal price while being even more protective...it makes literally no sense.
It seems to me that the issue is with the RAW rather than the role playing concept of a suit of armour that should be as rare as a powerful magic item
so, in order to craft adamantine/mithril/+1 suits of armour, you need to spend 250gp in raw materials and 250gp in "time investment", correct?
I'll probably do as follow, then:
of those 250gp, 100 will be treated as the cost of the base suit of armor. If your suit of armor costs less than 100, you'll get refunded (e.g. studded leather costs 45, thus you'll get a discount of 55gp), but if it costs more, you'll add the difference (e.g. a half plate costs 750gp, thus you'll get to add extra 650gp).
An adamantine full plate, then, will cost 1550gp + 250gp for an adjusted price of 1800gp while a +1 studded leather will cost 445gp.
Also I'd love to know how they ended up with the thresholds being 100 500 and 5k... now somehow it looks like that all the uncommon items must cost 500gp and not a penny less and all rare ones 5000...