How much does it cost to fix a broken back glass on an iPhone 12?

We are not Apple but rather other end users, and cannot do price quotes here.

This Apple repair site has an estimator:

iPhone Repair and Service - Apple Support

but does not include the iPhone 11. What it shows for iPhone 12 is between US$299 and $449 depending on the iPhone 12 version.

Apple does not show your phone to be either vintage or obsolete, so they should be able to work on it, but you need to ask Apple about the cost. It's probably going to be high.

You can get a repair estimate here: iPhone Repair and Service - Apple Support