Battery replacement iPad

Depends on the model. At least in the United States, if it's done through Apple it's either $99 or $119, plus tax and possible shipping if they have to ship the replacement directly to the customer from a warehouse.

Almost universally - Apple doesn't actually disassemble, replace the battery, and return the same iPad to the customer. It's a lot of work since iPads are essentially glued together. There are some hints that Apple might be doing same device battery replacements for limited iPad models, but apparently only at centralized repair locations and not at Apple Stores or Apple Authorized Service Providers. So pretty much any kind of iPad repair is just replacing the whole device with one with a new battery.

There are some caveats for a battery replacement service. They will check over the entire iPad for proper functionality and cosmetic/operating conditions. Any chip, crack, dent, bend, etc. will render it ineligible for a battery replacement service. So will any failure of the diagnostic that will be run. Something like the inevitable scratches around the Lightning or USB-C port is fine though. My battery wasn't quite degraded enough in the first place, but I was denied a battery replacement service in the future because an Apple Store employee laid it flat on a table and determined it was bent. He noted that in a database and told me that I wouldn't be able to get the battery replacement service in the future on the basis of poor cosmetic condition.

I did recently get a replacement under warranty for my iPad mini 6. Malfunction wasn't specifically disqualifying (it was still under warranty), but I did get a thorough check of the case and the glass, including laying it flat on a table to see if it was still perfectly flat. But the replacement device was more or less new. Seemed like a new case and definitely a new battery, but I had no way of knowing if the internals were new or one that came from a previously used device.

The cost varies by country/region. You can verify eligibility and local cost of the battery replacement service here:

iPad Repair & Service - Apple Support

For practical purposes, in lieu of replacing the battery in your current iPad, Apple will offer an exchange for a renewed/reconditioned device of the same model and specification. The cost of the exchange is the same as a replacement battery.