Cost of new phone

That depends what you mean by “replacement”. If you have AppleCare that comes with the phone and is free, and the phone fails within the warranty period (not from damage) Apple will replace it free. If it has been damaged there will be a replacement fee; for an iPhone 8 Plus that will be $399.

If you purchased AppleCare+ and it is still within the 2 year window a damaged phone will be replaced for $99.

If it is out of warranty completely then you pay the price of a new phone. If your current phone works you may be able to get a trade-in.

...And if the phone was lost or stolen and you don’t have AppleCare+ with the extra-cost theft and loss coverage, you’ll be acquiring a new or new-to-you phone at your own expense.