Armor of Combat, Ceremony, and Class
Some say that the age of chivalry is past, that the spirit of romance is dead. The age of chivalry is never past, so long as there is a wrong left unredressed on earth. - Charles Kingsley
Despite the common idea of shining armor and the heroic aura around it, not all armor shined quite so as one would expect. Most of the armor you see in museums and collections with the classic knightly sheen is likely ceremonial, preserved as for its ornate beauty and precise detail. This type of armor wasn't your usual battle armor, its ceremonial. Battle armor had grit, it was black and lusterless most of the time, and any battle armor you may find today would likely be in very poor shape! The armor a man-at-arms would wear could actually speak a lot about him, it could speak for his skills, the armorer, and prominently his social class.
Ceremonial Armor
A solid comparison that historians often make for a modern idea of how much different tiers of suits of armor would cost and who would wear them are cars. Let's hypothesize for a moment that an esteemed, wealthy, and high class medieval monarch or nobleman/woman were alive today. One of the most tell-tale signs of a level of social class for most famous people is the car that they drive, a monarch would want the highest quality vehicle, something crafted by only the best! They'd want a Rolls Royce, a Lamborghini, etc. Years ago, the same could be said for a suit of armor, an example the Wallace Collection provides of an excellent armor was Kolman Helmschmid and his family, working for various historical figures of wealth such as Charles V and Maximilian I. The masterful craftsman and his family even have their own section(s) in the book and museum itself! So, seeing a person attending ceremonies or walking about in gear from an armorer like Helmschmid wouldn't be unlike seeing someone driving around in a Lamborghini. From their armor or car, you could infer their social status or class. So, ceremonial armor is mostly what you see at museums, and the shiniest and most ornate types of armor. It was worn by kings and the exceptionally wealthy, rarely into battle and mostly for events, gathering, and recreational purposes. Despite this standard, the common folk could also get their own armor and foot soldiers could be equipped with so much as full sets of plate armor depending on their position, but it was of a much lower quality.
Combat Armor
As I stated in the preceding paragraph, the car analogy for armor can also give good insight on pricing, especially in that it was rather complicated. The question of "How much did a suit of armor of the medieval era cost?" isn't unlike "How much did a car in the 2000's cost?", as there could be a multitude of different and correct answers. What kind of car, what manufacturer, what year, has it been use, can all apply to armor as well. Higher end ceremonial armor would cost a fortune, kind of like a Lamborghini. Some lower end field armor for a foot soldier might take a few years to save up for, not unlike you were saving up for your first car, maybe a Honda Civic. You'd take your savings over to a blacksmith or armorer of your choice and desired price range, get fitted, and request your suit of armor as to your own specifications and the limits and talents of your armorer. Combat armor usually was limited in its cosmetic appeal, as unlike ceremonial armor, it was crafted with only protecting the user in mind, not how visually pleasing it was (Not to say a grizzled and rough set of combat armor didn't look cool!). Pricing for armor was like pricing for a car, you get what you pay for and its all down to who you want making it and what you're willing to spend! If you've managed to find some old combat armor in a museum, then its probably dark and damaged, riddled with scuffs and scratches and missing many of the straps and bolts that kept it together originally, replaced or lacking.