Whilst a “battery replacement” service is offered, in reality, Apple offer a rapid exchange service. Qualifying devices are simply exchanged for a reconditioned iPad (that has a new battery) of the same model and specification. The exchanged iPad is yours to keep.
For and iPad that is assessed as having no other damage or faults, the cost in the UK is £99. If other damage or faults are present, the cost of the exchange rises to c.60% of the “as new” cost of the iPad. If the exchange occurs at an Apple Store, your iPad is usually assessed in-store - and the exchange cost finalised.
If instead you mail-in the iPad, then the assessment is made by Apple’s partner organisation that handle “repairs” - and it is their determination alone that fixes the final cost. In this case, the c.60% charge is initially levied - with the balance being returned if the iPad qualifies for the lower charge.
For this reason, many choose to use a physical Apple Store to initiate a battery [iPad] exchange or repair - as costs are known before parting with your device. Similarly, this route removes the possibility of dispute concerning damage or defects after the iPad leaves your custody.
Here is the UK Price List:
iPad Repair – Official Apple Support