I meant the no OS bare boards. The Blue Pill is just one example, I paid around $2 for some to play around with. You’d need to do some porting and a new controller board retrack
http://wiki.stm32duino.com/index.php?title=Blue_Pill
There’s the re-arm Mega2560 pin compatible board from Panucatt, same controller, still needs some software porting, more expensive ($45 ish)
http://www.panucatt.com/mobile/Product.aspx?id=37884
On the RT Linux side there’s the Beagle Bone Black