No.
No one armor piercing bullet, so there is no one cost. Most are inexpensive to make- less than $1 per bullet.
Depends on the thickness of the platinum, the speed and weight of the bullet, and what the bullet is made of. A.50 caliber armor piercing bullet will shoot through some thicknesses of steel or concrete.
No. Ball ammo is a lead round nosed bullet with a copper jacket. It will just flatten when hitting armor.
There are many materials with bullet proof properties including Kevlar, Lexan, Titanium, Steel and Carbon Composites. Keep in mind that "bullet-proof" is a relative term. Armor-piercing bullets are designed to be able to penetrate most of these so-called "bullet-proof" materials that will do a decent job of stopping normal (non-armor-piercing) rounds.
They usually are constructed with a hard core or bullet, usually using steel or tungsten penetrators.
It all depends on your definition of "armor" and "armor piercing".
This all depends on your definition of armor and armor piercing.
There are several different technologies with armor-piercing bullets. A common one is for the bullet to have a thin core of steel, or other hard metal. Typically bullets have soft lead surrounded by soft copper. When these bullets hit something, they deform, usually increasing their diameter as they penetrate. Most armor exploits this property. Steel will not deform like lead & copper, and is much more likely to penetrate deeper than a regular bullet.
An armor piercing capped ballistic cap is a variety of armor-piercing shell introduced in the 1930s.
You can buy armor piercing rounds for it, if you can find them.
Nothing is really bullet proof. Anything labeled as such can always be beaten by a faster and/or heavier bullet. Things can be armored, bullet resistant, impact resistant ASO. And the best stuff for getting through things like that are armor piercing rounds.
An armor-piercing composite non-rigid is a variety of large solid projectile.