The STM32F411 Black Pill MCU is $0.99 on AliExpress. It's got a 100 MHz ARM Cortex-M4, FPU, DSP SIMD, 128 KB SRAM, and 512 KB Flash. It can run FreeRTOS. By porting a shell and a few essential utilities to this set up, it could be a standalone system that can run rings round PDP-11/40 UNIX 6ed (Lyrons era powerhouse).
Another option is to run MIT xv6 on a $5 RISC-V SBC. xv6 is essentially a stripped down, ANSI C port of UNIX 6ed.
Of course, there are tonnes of cheap ARM and RISC-V SBCs running Linux and BSD. But they are as complicated as any modern desktop, so not suited for bare metal programming. Not surprisingly, those who play with such systems inevitably develop Python, JavaScript, webdev, and other bad habits.
