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.