What is the repair cost for iPhone 14 Pro Max back glass?

$549 without a valid AppleCare+ Plan or $29 with a valid AppleCare+ Plan.

See this link --> iPhone Repair and Service - Apple Support

It is $549 without AppleCare+ and $29 with AppleCare+ as reported here:

iPhone Repair and Service - Apple Support

You can check the back glass replacement cost here: iPhone Repair and Service - Apple Support

Scroll down, select your device make and model — the pricing info will be listed there