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

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

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.