Xiaomi A6 can often be found new around me for less than $100 each, they have good battery, good camera and good LTE modems. And there are cheaper options with inferior (but possibly sufficient) hardware. 2nd hand previous generation Xiaomi or Meizu are often $20-$40 when you can find them.

Isn’t as easy to find them in the US, but aliexpress and eBay are your friends.

Also note that the guy spent tens of hours of a highly competent engineer on this project. Even if he had used iPhone XSs, that wouldn’t have been the expensive part of his project...