How much does it cost to change the battery in a iPad Pro, 3rd Generation?

You can establish eligibility and cost of a battery replacement in your country/region here:

iPad Repair & Service - Apple Support

For practical purposes, in lieu of changing the battery in your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for the battery swap.