Actual costs Gallon of Gas (California) for March 31, 2008:

Distribution Costs, Marketing Costs and Profits $0.06
Crude Oil Cost $2.42
Refinery Cost and Profits $0.48
State Underground Storage Tank Fee $0.01
State and Local Sales Tax $0.27
State Excise Tax $0.18
Federal Excise Tax $0.18

Retail price $3.61
Net Profit $0.49

Profit Margin 13.5%

If you want to talk about actual "costs" it's an entirely different
animal. Crude prices are drive by commodity rates. Pump prices include
that and taxes, and different levels of profit and labor costs.
If you assume reasonable labor rates along the way, .50 gallon is
probably a realistic cost estimate.