Oil company profits go up and down, and your overall estimate for profit margin is probably close to the truth if one averages over a ten-year period or so.
My figures used the Exxon-Mobil Annual Report for just one year, 2006, a year with rising prices. If one would look more deeply into the Company. allowances for contingencies, future taxes, repayments planned on old debt, etc., the earnings per share would be much less. After shareholder dividends are paid, the amount of new funds available to the company would be relatively small. I am not an economist, and if results are not in words of one syllable, a get boggled. My figures trying to relate to cost/bbl could be way off.

Some more figures- (selected)
Royal Dutch Shell Annual Report, 2007
Revenue 366,000 million
Earnings- $32,000 million
Earnings per ordinary share- $5.00
Costs and Expenses-?
Dividends paid, common shares- $1.44
Capital investment- $27,000 million (includes depreciation, etc.;, $19 billion actual)