How much does a new battery cost for a iPhone 8plus

A battery replacement for iPhone 8 Plus will cost you $49 USD.

Source: https://support.apple.com/iphone/repair/service/battery-power

Apple announced a pricedrop for the cost of replacing out-of-warranty battery replacement from $79 to $29 (approx Rs 1,800). The offer is valid for anyone with an iPhone SE, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus. But you have to conform from apple service center