The cost of production seems to vary widely...
Some countries in Opec can remain profitable even at $15 per barrel
http://www.maritime-connector.com/NewsDetails/2377/lang/English/OPEC-and-Production-Cuts--Why-Now-s-the-Time-to-Buy.wshtml
I remember in 1998 during Asian crisis when oil was as low as $12, one
of the biggest oil companies(forgot which one)in the US claimed that
they still can make money even if oil is $10.