http://www.progress.org/gasoline.htm

This article at $12.75 from May 2011:

http://www.triplepundit.com/2011/05/increasing-gas-prices-su...

Subtracting the $4 price of gas is what brought me to $8, conservatively. To be fair, some of the price paid at the pump is tax, but according to the previously linked wikipedia article that's 12%, so $0.48 for a $4 gallon.