how much does it cost to replace the battery of ipad pro

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

iPad Repair & Service - Apple Support

Apple generally considers batteries fit for use until they drop below 80%.

According to Apple:

Battery ➢ iPad Pro ➢ 5th Generation = $119.

▶︎ Click Here for Service. ◀︎

The battery replacement of an 11-inch iPad Pro model is £165/$202USD. The 12.9-inch model is £195/$239USD.