what is apple watch battery service cost in USA?
for batteries for apple watches s5 and s6, both 40mm cell, in USA? on the phone im told $99 each but when I get to the store they say $300 each.
Apple Watch Series 5, watchOS 10
Have a look at this Apple Watch Service page and the "How much will it cost" section, where you can get an estimate for your model:
Apple Watch Service and Repair - Apple Support