There is not a lot of pricing room between a $1 board with 10c 48 MHz 2 KB RAM chip (as here) and a $5 board with 1 GHz 64 bit 64 MB RAM board running Linux (Milk-V Duo).
That gap is pretty much filled by boards using e.g. the ESP32-C3 with 400 KB RAM, running at 160 MHz. You cvan find quite a few of those for around $1.80.
As pointed out in my other comment, you can add vast amounts of extra functionality for very little cost, up to a Linux-running board with 64 MB RAM for $5.
But that's not the point. The point is: what is the cheapest stand-alone usable computer you can make?
"Usable" in the same sense as a ZX80 or Altair 8800 or Apple I of course, not in the sense of a modern computer.
If you pay 50c for the CPU instead of 10c then you just increased the total cost by 50%.
PS: If I’m not mistaken, this would mean a minimum of 256K video memory - so, say, another 256K for general RAM. So perhaps a 512K or 1MB system.