My thought was an additional 500 gp on top of the normal armor cost.

The removal of disadvantage to stealth is incredibly powerful for the heavier plate wearers. In my noob DM days, gave it to our paladin within the third session in a two part dungeon and didn’t realize the insane boon i’ve given him as now he can almost be as stealthy as the rest of the group and had 20 AC with a sword and board :( makes for a lot of modifying creature’s perception and to hit modifiers to be able to have a chance at hitting this silent moving tank of death. I would suggest maybe higher than 500 GP for at least the heavy armor and higher AC mediun armor or even holding off until higher levels of making it available as a drop or as a purchase in a store. That way they’ll be fighting creatures that actually have a chance to hit 20 AC on a regular basis.

Would this stack with +1,2, and 3

Quote from gaber11979 >>

Would this stack with +1,2, and 3

 I hope so

When it says "Mithral is a light, flexible metal" would it mean it is lighter then the normal armor? It also says it's flexible, does that mean that you might be able to add more of your dexterity bonus? I am going to ask my DM about this.

Mithral does not seem to be inherently a magical metal, just uncommon. However, there does not seem to be a specific value attached to the armor for that rarity. When it comes to value for sale, the DMG (pg 135) puts Uncommon items at 101-500 gp.  The question seems to be, would you add that number to the value of the armor, or would that number replace it?

Quote from pkline5 >>

Mithral does not seem to be inherently a magical metal, just uncommon. However, there does not seem to be a specific value attached to the armor for that rarity. When it comes to value for sale, the DMG (pg 135) puts Uncommon items at 101-500 gp.  The question seems to be, would you add that number to the value of the armor, or would that number replace it?

Mithral is inherently magical, which is why this is listed under magic items. It is an uncommon magic item. 

Magic Item Rarity

"Each magic item has a rarity: common, uncommon, rare, very rare, or legendary."

According to the table there, uncommon magic items are valued at 101gp - 500gp, but that doesn't mean your players can just go to the store and buy a set of mithral plate for 500gp. That's just a general rule of thumb for how much an item is worth, and is usually there to tell the players how much they can sell a magic item for, not how much they can buy it for.

It depends on the magic level of your game. So DMs like to play games that are very high magic where magical items are plentiful and shops sell them. That's totally fine. But in most games, magic items are rare, and you will have difficulty buying them. I think I would say you could after a week of investigation find a master dwarven armorsmith who will be willing to craft mithral armor for you. Either you need to bring him the raw mithral, which he will alloy with steel armor, or you're going to have to pay a lot.

Given that armor has value, I would say add 101 - 500gp to its normal value for the mithral version. But a person trying to sell an item is definitely going to mark that up. A full set of plate is normally 1500gp, so I would roll to see how much to add, then mark it up by 15%. 

The most important thing to consider is what fits the narrative and what is fun. If you think it's fun to let your players go buy mithral armor from a shop in a city, then by all means. Personally, I think magic items are more interesting when earned via a quest or adventure, or in this case seeking out someone who can craft it. In Forgotten Realms, the art of smelting mithral and alloying it with steel is a carefully guarded secret of certain dwarves, so you're unlikely to find a full set except maybe being sold in an auction by an eccentric collector, or collecting dust in some dwarven or elven ruins.

I think you are selling it too cheap.  Not just any smith can forge mithral items and unless you are in/near a larger city, or dwarven clan, it would probably take longer than a week to find a smith.  Then I would consider that mithral should also be much harder to work with than steel, let alone creating the alloy first.  I'd double the value... minimum and require 6 months to make.

Personally I wouldn't allow mithral armor to be easily purchasable at all in my games, but it depends on the kind of game you're running. You are already making assumptions about it based on one setting you're familiar with, which isn't strictly helpful for the discussion at hand, although it is valuable input for some settings.

RAW based on the utility it offers and its rarity, mithral armor is worth between 101gp and 500gp plus the base cost of the type of armor. So for full plate that's between 1601gp and 2000gp. And I didn't make that up. It's straight out of the DMG.

In my game, there are very few people who can work mithral, and they are unlikely to offer their skills for cheap, but every game is different, so you have to use the rules to give people a baseline.

Well, the DMG is just a guide.

Is this Mithral a whole set of armor  of itself or an attachment? . My DM and a lot of they friends say this is attachment ,but it not make any sense. Like heavy armor + Mithral attachment become light with no disadvantage ???

It'd be heavy armor made from mithral.

Quote from WillRosewall >>

Is this Mithral a whole set of armor  of itself or an attachment? . My DM and a lot of they friends say this is attachment ,but it not make any sense. Like heavy armor + Mithral attachment become light with no disadvantage ???

 It'd be the armor, but made from mithral... Not an attachment to the armor...

I guess it could be the armor lined with mithral, seeing as mithral has magical properties, it changes the armors properties as a whole when you line it with this.

It would generally mean the armor is easier to wear, especially when the line immediately following it talks about mithral chain shirts and breastplates being wearable under clothes.  Which is pretty awesome by itself in a lot of circumstances, don't underestimate the benefits of concealable armor.  I'd likely knock the weight down myself, but allowing more Dex mod is something you normally have to take a feat to do. 

What's the ruling on mithral armor and, say ... a rust monster?

Based on the rules in the PHB and DMG, Mithral is immune to the effects of a Rust Monster. Mithral is counted as a magical armour, and Rust Monsters only affect "non-magical metals". 

I am trying to find out how heavy mithral is and for some reason a Chain Shirt weighs 20 pounds while a mithral Chain Shirt is 20 pounds, so how is mithral lighter than things like Iron if the armor weighs the same?

In an earlier edition of D&D, an official source book listed mithral as costing 2,500 gp per pound. Even if we suppose that mithral armor is only 10% the weight of normal armor, it's still far more expensive than that. When I've used it, I've also ruled that it requires special equipment to work at all. Magical forges powered by bound fire elementals, magma forges, runic hammers, etc. That said, a spellcaster with Fabricate, could - in principle turn a raw lump of mithral into anything they wanted - assuming they knew enough about armorsmithing to make it. Fabricate can be an absurdly powerful spell if the DM doesn't put some kind of bounds on it.

It weighs half as much as items forged by steel.

https://forgottenrealms.fandom.com/wiki/Mithral