So my idea is to have you craft with exotic materials for neat effects to help lower the need for magic to have unique gear. One example is rubber armor the cost is 10 times that of normal light armor but it has immunity to lightning and -1 ac I would like some recommendations to make it even better

The classic is to use monster parts. Like armor out of dragon scale that offers resistance to whatever element is tied to that kind of dragon. Leather from a displacer beast that gives you some kind of limited displacement. You can even use it to make existing items -- ogre parts for gauntlets of ogre power, for example.

There's all kinds of unusual metals you could use, just make up some name and say its from a meteor or the deepest part of the underdark. It's super dense, so it gives a +1 to AC, but that density means its so heavy that it means a -10 feet to movement, no matter your STR (though I guess dwarves would love this stuff).

Weapons can work the same way, a horn from that same dragon that makes all arrows fired with it do elemental damage of the appropriate type. My gut says you don't need to give this a drawback. More often than not, damage type doesn't really come into play, I don't think. So few monsters are vulnerable to any given damage type, and they are kind of balanced by ones that are resistant, that I don't think it would really be too OP. But that's just a gut feeling, someone on here may come along and note something I'm forgetting.

A storm giants hair to string a bow that give a +1 to hit and turns arrows electric, but has such a hard pull it can only be used by someone with a 15 STR. That star metal again that's really dense gives you a bonus to damage, but a -1 to hit because its so unwieldy.

And I you want to go really extreme, you can always throw in fantasy-style weird stuff, like the breath of a flower, or the kiss of a wolf, or things that finding just become a quest all by themselves.

I'd also consider things more universal than your example. In that case, I know lightning attacks are so rare that there's no way I'd take a -1 to AC (all the time) at 10x the cost for immunity to something that will almost never come up (unless you have a super-lightning campaign or something). Things that are always on (like AC bonus in exchange for speed) start to make things more tempting and the trade offs more interesting, at least personally.

It won't significantly affect the game if you replace magic items with items crafted from exotic materials (if the exotic materials are non-magical the item will function inside an anti-magic sphere and won't register on detect magic, but neither of those are really dramatic balance factors).

Thanks I’ll lower the cost to something like 5 times and I was honestly thinking the rubber armor for Npcs rather then players for world development.

Here’s what I have so far might rebalance prices later 

Crafting material guide. Cost, Uses, and Location.

Mundane materials

Steel, Bronze, 1 gp per Lb cost mod 1t, regular, crafted.

Iron, Copper, lead, 5 sp per Lb cost mod 0.5t, Heavy (1.25 Item Weight) weak (on weapons if you roll a 1 their is a 50 50 chance it will break but with armour if you get hit by a 20 same 50 50 chance )almost everywhere if its lead it can be enchanted to 1.5x the ac of the armor for 3 rounds 10 lvl this is a joke about bullet sponge and eat lead

Uncommon mundane materials

Silver, 50gp per Lb cost mod 50t, -1 if hitting enemies or getting hit by enemies if it’s for a creature vulnerable to silver it’s a +2 for attacking and getting hit and a 19-20 is a critical hit for you, found uncommonly in mines

Glass, 5cp per Lb cost mod 0.2t, only usable by weapon -3 weapons deal 3x critical damage fragile(if you roll a 5 or lower on attacking it shatters) enchantment you do a critical hit but the next strike against you is a critical hit 1/long rest 5 lvl 1 day enchantment, found anywhere there’s sand.

Obsidian, 5 gp per Lb cost mod 4t, Light(0.75 item weight) fragile(a 1 to hit for weapons or a critical hit against you for armor) enchantment volcanic wrath summon a fire ball (fireball spell dc 13) to attack your foes 1/long rest 7 lvl, found near and in volcanic places.

Rare mundane materials

Rubber,5 gp per Lb cost mod 3t(for weapons Add it to the material your using) for armor 5t light only, not suitable for weapons only handles and armour, Lightning resistance( with weapons you have resistance to lightning damage with armor it’s -1 but your immune to lightning damage, only available to purchase on technology advanced places

Uncommon wood

Ebony wood,50 gp per pound 25t weapon  handle, ebb the flow(take 1d8 hp from one willing creature to another, action) unearthly resilience(if destroyed it reforms the weapon over the course of 1d8 hours, found as the heart of Shadowfell trees.

Rare wood

Whisper wood, NA no place where it’s sold, can only be used to form clubs or a quarter staff unsettling whispers (while held you can hear whispers that if you listen can give you advice however everyone around you 5ft radius must make a dc 5 wisdom saving throw to not be frightened ends when out of radius) insane shell (while holding the object if your mind is read the creature reading it will not be able to read it and once they sleep will experience nightmares that remove the benefit of the rest and they take 2d8 physic damage) lost sanity (from contact with the strange substance you have a mild madness your dm decides), found in the outskirts of the realm near the far realm