Raspberry Pi Pico: $4

 The Raspberry Pi Pico is a tiny $4 microcontroller running off the company’s very own chip - The Verge

New Raspberry Pico Micro-Controller Brings Versatility and Small Size Factor

The Raspberry Pi Foundation recently released the Raspberry Pi Pico, a small (21mm x 51mm), inexpensive ($4) micro-controller board based on a custom-designed RP2040 chip. The Pico can be programmed in C and MicroPython via USB. The RP2040 has two ARM cores clocking at 133MHz, 264KB internal SRAM, and 2MB QSPI Flash to connect external memories. The Pico enables a large range of applications by providing a wide range of flexible I/O options (I2C, SPI, PWM, 8 Programmable I/O state machines for custom peripheral support).