How much will it cost me to repair my front and back screen for iPhone 8 Plus

I posted a link to the current online repair prices from Apple. As far as I or anyone else knows, those are Apple’s prices in the USA.

I‘d also post this - https://www.apple.com/support/products/iphone.html

Repairing the front and back glass requires an out of warranty replacement, not merely a screen repair.

The repair prices that you posted were for the screen only. Since the OP identified both the screen and the back glass, that requires an out of warranty replacement. Apple cannot replace the back glass, they have to replace the entire device, thus the out of warranty cost that Michael quoted.

In the USA an out of warranty replacement with AppleCare+ would be $99 and without AppleCare+ it will be $349

iPhone Repair - Official Apple Support

If you have Apple Care it is a 29$ repair charge. You get 2 accidental repairs with Apple Care then it will be full price.

Take a look at this