Waterproofing hasn’t been relevant to batteries since they were made internal.
I just checked, and a replacement battery for my 2024 HMD Skyline goes for $15.62 plus another $3.25 for the consumable adhesive pad and shipping [0]. In practice, you might also need to buy tools, which can further drive up the cost.
Finding that source took a bit of knowledge. If you go by what HMD tells it’s US users, you would look on iFixit, which would be $30 for an entire kit, which includes the battery, adhesive, and tools (Just the battery and adhesive would run you about $28).
[0] gsmnet.com/…/hmd-skyline-battery-hba4633aa-servic…