I think the Luckfox Pico series is the lowest cost ARM-based board you can buy (that runs Linux) at the moment. Even the Pi Zero is $10. Prior to this, it was a board based on the Allwinner F1C100, but I don't think anyone made and sold a dev board except for a DIY business card [0].
[0] https://www.thirtythreeforty.net/posts/2019/12/my-business-c...