There are several cheap ones listed here: https://riscv.org/exchange/ https://riscv.org/exchange/ such as the $30 Sparkfun Red or the $20 Lofive boards. Those

There are several cheap ones listed here: https://riscv.org/exchange/ https://riscv.org/exchange/ such as the $30 Sparkfun Red or the $20 Lofive boards. Those are for running an RTOS, not Linux, but they compete with Arduino, mbed, teensy, and other ARM Cortex M series microcontrollers. A price target of $10 is something you'll only hit with massive scale-up.