The 'DevKit-C' breakout boards are $10 each from authorized distributors; they're a little large and you could probably find variations for closer to $6-7 if you went looking, but they're good for general-purpose stuff.

It seems like small power-efficient microcontrollers have made enormous progress over the past decade, and we have an opportunity for the PC revolution all over again. The ESP32 is a little bit power-hungry, but it's also a proper CPU; it has two 240MHz cores, a few hundred kilobytes of RAM, an extra low-power core that can wake the main system up from sleep, and an MPU that can keep track of 8 process IDs per core including 2 privileged ones. People used to make do with less. It also has WiFi and Bluetooth and these days you can even write programs for it in Python.

I'd love to see more people thinking about the problems in their lives with relation to a $20 bin of modern parts. Sensors, displays, and actuators have made similar progress, and the possibilities are endless. But the pool of people trying to realize them is finite and way too small.

I use the LOLIN32 boards for this.