Actually, going by historical examples, plate mail was actually quite common in the deserts of Arabia and northern Africa. Admittedly, Ottoman Mirror Armor isn't aesthetically the same as 15th century panoply but at a functional level, it is at least very similar. Next thing to remember is, if there is significant interaction between nations, there will be trade and the sharing of technology. While the use of firearms is apparently uncommon enough to justify its lack of trafficking, more common weapons, like longbows and crossbows, swords, spears, etc. would still result in the adoption of armor with very similar natures. For example, upon contact with Celtic warriors, Rome, seeing the superiority of Celtic technology to Greek designs, began to adopt Celtic hardware: helmets based on Celtic designs, Shields based on the the Battlesea, the adoption of mail for use by heavy infantry and the introduction of the Spatha all trace their origin to the Gaelic conquest.
As for the matter of price, keep in mind, not all armor is created equal. While your average soldier probably could not afford a full suit of tempered plate, softer mild steel armor was still quite affordable. And of course, then there's lamellar armor, which can be even cheaper, probably the prefered armor of your peasant levies in the form of the brigandine. Another thing to remember is that, since armor had to be tailored to fit the wearer, it was not uncommon for a soldier to start his career with just a helmet and shield and then build his suit over time. And lastly, keep in mind that armor was not considered a frivolous expense in a time when fighting was not uncommon. Think of it this way: You are an average peasant; you know that some day, you are going to be drafted into the army; you also know that you will only be able to bring your own own property with you into battle; does draining your life savings seem like an absurd cost to guarantee your own survival? And of course, cost is primarily tied to quantity rather than quality. If all I can afford is a suite of torso protection (cuirass, plackart, backplate), why not buy that instead of wasting money on a full body mail harness that turns out to be worthless at stopping these newfangled warhammers that everyone is using.
Of course, magic throws a new wrench into the works as defensive magic may be able to match the protection afforded by plate armor, or offensive spells capable of becoming as effective as firearms, nullifying the need for powerful ranged weapons to a degree (after all, a non-mage in a mage society could still need to fight at range with a bow). So, I guess its not so important that every country has access to the same armor and weapons, but they must at least be able to rival each other in some way. After all, Warfare is the most primal and monsterous form of competition with the highest stakes imaginable. Everyone inclined to participate in such an endeavor would want as much of an advantage as possible and when that arms race is not acknowledged, it can strain believability.
At least, that's my take on things. As I said, you guys don't need to change to accommodate my style; I just want to know if these questions are antagonistic to what the site as a whole wants and if my understand of medieval and early modern warfare is going to affect other people's enjoyment of any potential future RPs...