Cheap ARM0 boards on ebay
There are some super cheap STM32F030F4P6 ARM CORTEX-M0 boards available via ebay/Ali Express. They cost about $1.50 (incl shipping) and are usable via PlatformIO with Arduino software.
They are a bit limited, having only 16kB flash and 4kB RAM memory, but usable for a lot of projects.
This board cannot be programmed via USB, although there is a connector it is only used to provide power. You need a programmer, i use a ST Link v2 compatible programmer obtained via ebay, costs about $2.50.
Connect the programmer to the board, you need 5 wires:
Set the Boot0-Jumper to "GND"
VS Code with PlatformIO
I use Microsofts Visual Studio Code on Linux (it's available for Windows and OSX too). Install the PlatformIO extension for Visual Studio Code and you are ready to go. This makes life much easier because PlatforIO takes care of installing all the libraries, SDKs and so on which are needed.
For the board choose Demo F030F4, Arduino for the environment. Plug in the programmer and put a blink sketch into "main.cpp":
// initialize LED digital pin as an output.
// turn the LED on (LOW is the voltage level)
// wait for 10ms
// turn the LED off by making the voltage HIGH
// wait for two seconds
Compile and transfer the program to the board and enjoy the warm glow of the LED blinking...