It is estimated that a total of 5 million barrels of crude oil have leaked from BP's Deepwater Horizon site, making it the worst oil spill in history. And as of last night, the company's market capitalization was down a total of $62.5 billion when compared to before the disaster. The loss was even bigger before the rebound, at $106 billion, see chart below.
Price of BP Shares
So what's the cost of a barrel of oil? For BP, it's definitely a case of crying over spilled crude: doing the division it comes to $12,500 per barrel - and it was even more before the bounce, at $21,250/bbl.
If we go a bit further and amortize the loss over BP's entire crude oil plus equivalents production for a whole year, it comes to $43/bbl. And if we go all the way and amortize the market cap loss over their entire proven reserves, it comes to $3.40/bbl. (Data from BP's 2009 Annual Report.)
Choose the number that best reflects "reality" in your view - it's only money, after all. However, it does put a number on one of those "external" costs for oil.
Now... what's the cost for the Iraq and Afghanistan wars?... hmmm.