Battery replacement for iphone 6s How much dose it cost?

My mom has a iPhone 6s and I just checked the battery health, and saw it says service, and the phone is out warranty I was just wondering how much it would cost to have apple replace the battery, I we do the battery replacement I'm just kind of worried that Apple might not have battery for the 6s anymore since the iPhone 6s is discontinued. Are better off just get a newer phone.