I've just designed an ARM Cortex M4 board, similar form fact to the Teensy/Pro etc. Cost of parts, maybe $30-40 plus the agro of getting prototypes made, getting them working etc etc. I'll be $1000 in the red before I even think about selling one, $500 if I just use them myself.
That's a lot of $2 clones, or even $20 Teensys.