When is iPhone battery percentage low enough to warrant new phone purchase

Honestly, that is completely up to you, and what your uses for the phone are. Apple has settings that help prolong the life of the battery, but it generally slows down the rest of the phone.

Since you are really only worried about the battery of the phone, it makes sense to just have the battery replaced. It costs around $89 USD per Apple's official repair website, or free if you have AppleCare. But keep in mind, it is completely up to you.

Apple recommends replacing the battery, when battery health dips to 80% (or less). It's completely up to you if you want to replace the battery or buy a new iPhone.