Quark D2000 Dev Kit from @Intel is $15, Arduino Shield-Compatible
The Quark D2000 comes with an on-board accelerometer, temperature sensor, Arduino Uno-compatible shield interface, and yes that’s a CR32 coin cell slot as this dev kit is rated to operate at a mere 2.0-3.3V. However some users are upset with the kit, specifically the 32KB of instruction flash; that said, Mouser stock is currently at 0 for a few days now, so the board is clearly liked by some at only $15.
Make a robot friend with Adafruit’s CRICKIT – A Creative Robotics & Interactive Construction Kit. It’s an add-on to our popular Circuit Playground Express, FEATHER and other platforms to make and program robots with CircuitPython, MakeCode, and Arduino. Start controlling motors, servos, solenoids. You also get signal pins, capacitive touch sensors, a NeoPixel driver and amplified speaker output. It complements & extends your boards so you can still use all the goodies on the microcontroller, now you have a robotics playground as well.