I have an IPhone 7 Plus. and my battery function is significantly degraded. How much to replace the battery?

Depends on your location and whether or not you choose to go through Apple. Some Apple Authorized Service Providers in countries where Apple provides service may charge a different price than Apple. Also, some AASPs will charge a diagnostic fee on top of the service price.

But as stated by another poster, it's $49 plus tax in the United States through Apple.

If you get it serviced by Apple the cost is quoted as US$49

See iPhone Battery Replacement