Used old Nokias (no, really); the highest cost would be moving them to-and-fro, and battery replacements. I still have a fully functional 3410, some ten years old. Battery is gone of course, but apart from this the phone took quite some licking, yet keeps on ticking ;) And of course the venerable 3310s - built like tanks, apart from the clip-on plastic covers.

Calls? Check. SMS? Check. English? Check. Not sure what you mean by "GSM" - it uses the 900 MHz and 1800 GSM ranges, if that's what you're asking

Alas, they don't make cellphones like they used to.

If you're thinking in terms of "gotta be a new phone", you could probably get <15 USD if you licensed those and had them built in huge batches.