How expensive would it be to replace the battery in a MacBook Pro (Retina, 15-inch, Mid 2014)?
MacBook Pro (Retina, 15-inch, Mid 2014) is running fine, except it indicates that my battery needs service. How expensive would a replacement be?
A list of out of warranty cost from Apple can be found here: Mac Repair - Official Apple Support
Going by what you are saying it should cost you $199 for your repair if you went to Apple for repairs. You can also try to find an Apple Authorized Repair Specialist.