Stm8, $1.28 http://m.aliexpress.com/item/32351992629.html
Stm32, $3.06 http://m.aliexpress.com/item/1568685935.html
Edit: my phone did a stupid.
Most "shields" and devices are tested and libraries made easily available for Arduino/Atmel 324P. Going to ARM means having to rebuild some of your toolkit, or finding it.
But again, it's ARM. And the "more expensive one" is 32bit.
Stm8, $1.28 http://m.aliexpress.com/item/32351992629.html
Stm32, $3.06 http://m.aliexpress.com/item/1568685935.html
Edit: my phone did a stupid.