How much does it cost to replace a battery for an iPhone 8 Plus?

We are end users like you and cannot quote prices, which vary with where you are in the world. Please start here:

iPhone Battery Repair & Replacement - Apple Support

Hello Myphone863!

If you scroll down for a bit on the page Allen Jones provided, you can enter your iPhone into the Get Estimate tool to get an estimate on the cost of replacing your iPhone’s battery.

I hope this is helpful. Good Luck!