What is the cost of a battery replacement for an Apple Watch Series 7?

If you fill in your Apple Watch model on this Apple Watch Service page, you can get an estimate for the battery replacement. It looks like all Series 7 battery replacements are about $99:

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copied from: Apple Watch Service and Repair - Apple Support

Depends if you are replacing the battery through apple or a third party. If you set up the watch in the last 2 years, and the battery health is less than 80% ( you can check in settings) you might be able to get your watch battery fixed by apple even if you don’t have apple care plus for a fee. (Fee is around $99 USD if you do not have apple care.) If you do have apple care and your battery health is below 80% you could possibly get it replaced for free.

That's a question to directly ask Apple. Volunteer users here don't usually have that kind of information.

Apple Watch Service and Repair - Apple Support

M1 Mac mini, macOS 15.3.1;  Watch, series 10 GPS-only, watchOS 11.3.1; iPhone 16, iOS 18.3.1; TV 4th Gen, tvOS 18.3; HomePods, audioOS 18.3 and Airport Express w/ Airplay 2