Hi Stefaan.
http://www.opec.org/home/basket.aspx
Yearly average Oil barrel prices were below 80$ before 2008 (see link above).
Somehow, I doubt that production cost followed the curve of the market.
But of course, it would suit the oil companies well to suggest the cost of production is higher than it actually is. Simply makes them look better (or not as bad).
Extraction from tar sands, like they do in Alberta, Canada, is quite expensive though.
Alberta is an important oil provider to the US.