They are a bit more expensive than the chip in the Arduino, but well under $20 per chip. You can buy very nice evaluation Cortex boards for ~$10 from ST and Freescale[1,2].
Incidentally, Arduino now makes a board that uses the Arm Cortex chip: the Arduino Due[3]. I believe it's around $30.
1. http://www.element14.com/community/docs/DOC-46626/l/element1...
2. http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/PF252...
[0] https://raw.githubusercontent.com/micropython/pyboard/master... [1] http://www.alldatasheet.com/datasheet-pdf/pdf/510587/STMICRO...
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