What is the price of a MacBook battery (Earlier model)?

IIRC, the Apple price is about $150 US for a MBAir Battery repair:

Mac Repair & Service - Apple Support

If your Mac is no longer supported by Apple, then you can get a third party battery from OWC or iFixIt since the 2017 & earlier MBAirs are very easy to replace the battery. However, the quality of third party Lithium batteries is extremely poor so it is best to go with an official Apple OEM battery replacement if possible, otherwise go with the two third party vendors I listed.

How early is "early"?

To identify your model MacBook: Identify your MacBook model - Apple Support

Have you checked the About This Mac dialog in the Apple () menu? The model name and hardware release year / time of year are often there. There is also the System Report (accessed from that dialog) where you may see further identifiers (like the Model ID or EMC, though those are not always unique).