Here are some of the factors that can affect the cost of an iPhone 8 back glass replacement:

• The location of the repair service: In general, repair services in urban areas will charge more than those in rural areas.
• The type of repair service: Some repair services only offer mail-in repairs, while others offer in-store repairs. In-store repairs typically cost more than mail-in repairs.
• The condition of the phone: If the phone has other damage, such as a cracked screen, the repair service may charge more for the back glass replacement.

If you are considering having the back glass of your iPhone 8 Plus replaced, it is important to get quotes from several different repair services before making a decision. You can also do the repair yourself, but it is a difficult and delicate process.

Here are some steps on how to replace the back glass of an iPhone 8 Plus:

1. Heat up the back of the phone with a heat gun or hair dryer. This will loosen the adhesive that holds the glass in place.
2. Insert a suction cup into the gap between the glass and the phone.
3. Pull up on the suction cup to create a gap between the glass and the phone.
4. Insert a thin pry tool into the gap and gently pry the glass away from the phone.
5. Be careful not to damage the camera or other components.
6. Once the glass is removed, clean the adhesive residue from the phone.
7. Apply new adhesive to the back of the phone.
8. Align the new glass and press it down firmly.
9. Let the adhesive cure for at least 24 hours.

If you are not comfortable doing the repair yourself, it is best to take your phone to a qualified repair service.