Prepaid mobile carriers offer "free" phones with one month of service. $25 a piece for Samsung a16 4-pack from Metro, if you only want 1 you can do $45 for a moto g power (8gb RAM) from Straight Talk. I've been using these as alternatives for SBC projects, cheap, battery and screen can be nice, most GPIO needs can be achieved by slapping on a $2 ESP32 clone over usb-serial. The supply will probably dry up soon, I think they might be eating a loss on just the RAM BOM.

Won’t you have tons of driver problems though vs an RPi? It would be super convenient packaging as an SBC though !

45 is still not 20 bucks.

Check your local pawn shops/thrift stores/antique stores/craigslist/kajiji/ebay.

There's a good chance you will find an old android for about $20 that still works.

For instance: https://www.ebay.com/itm/336183863624

Took me less than 1 minute to find.

In Kenya you can get a Safaricom Smart Neon 4G phone for $22. It's actually not bad at all. It is an operator sponsored phone, but there is no monthly plan. Safaricom sustains its digital payment system (mpesa) with it.

There is this called called "Obama phone" (free cellphone and free service for low income people).

You can get one if you know some people

https://en.wikipedia.org/wiki/Lifeline_(FCC_program)