Damage of back body

The back glass on iPhone is fused to the phone and cannot be repaired/replaced. As such, the entire phone must be replaced. You can purchase an Out of Warranty replacement for about 60% of the cost when new, or if you purchased AppleCare+ when you bought the phone and the coverage is still active, a replacement phone will cost $99. Without AppleCare+ the replacement cost is $399.

See this link --> iPhone Repair - Official Apple Support

What’s the price of full body case iPhone xs

I don't know what you mean by full body case? If the back glass on an iPhone has been cracked, you have to pay for a replacement phone.