The answer depends a lot on what you need it to be able to do.
Make calls? Support the latest banking/travel/communication app updates? Run any old VOIP app in a pinch as a true "backup phone"? Simply connect to wifi and browse the web? Just play basic media?
The older they are, the less they will have support for recent app versions or "modern" websites.
But there are some good old iPhones under $100. Whether it does what you need it do, to serve as a backup, depends a lot on what sort of functionality you are backing up.
Go on Swappa, sort unlocked iPhones by low price, pick one with good photos and description in your budget. Ask an LLM what limitations it will have, since at that price it will probably have some compared with what a new phone can do.
