iPad battery

iPad Repair - Official Apple Support

if it's a eligible model 99$ it seems

Apple doesn’t offer a battery replacement service - but for qualifying devices, it does offer an exchange programme where your iPad is exchanged for a reconditioned iPad of the same model and specification. If the iPad is otherwise functional and undamaged, the cost of the exchange is equivalent to that of a battery replacement .

If the iPad has other damage or defects, the cost if out-of-warranty exchange is typically c.60% of the “as new” price of the iPad.