cost of battery replacement in iPad

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

iPad Repair & Service - Apple Support

You might choose to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - prior to committing to a battery replacement or other repair:

Genius Bar Reservation and Apple Support Options - Apple

For practical purposes, in lieu of servicing and returning 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 battery replacement.

If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.

The cost to replace your iPad Pro battery varies from $119-$199, also Apple recommends you replace your battery when the battery health reaches below 80%. If you have Apple Care+, which means your battery can be replaced at no charge if your product is tested and its battery retains less than 80% of its original capacity.

iPad Repair & Service - Apple Support