Hours after admitting the failure of its heroic attempt to use a four-story bottlecap to stanch the flow of oil into the Gulf of Mexico, BP (NYSE: BP) updated shareholders on what its efforts have cost them so far: $350 million.

Meanwhile, analysts are wiping sweat from their green eyeshades, and furiously punching numbers into their spreadsheets, trying to gauge the ultimate hit to BP. So far, their estimates range anywhere from a few hundred million dollars (been there, done that already) to $12 billion. And then there are the "X" factors: litigation costs, government fines, punitive damages.

Yet even after you tally up all these big numbers, there's one more that looms largest of all -- the $30 billion in market cap that BP has already lost as a result of this disaster.

BP's not the only stock that's been torpedoed by the Deepwater Horizon, of course. Anadarko Petroleum (NYSE: APC) owns 25% of the well in question. Transocean (NYSE: RIG) owned the rig itself. Halliburton (NYSE: HAL) and Cameron (NYSE: CAM) are also taking hits for their roles in the disaster. Those four don't even fit on the chart ...

But maybe they don't have to. Subtract the first three columns from the last, and it looks like there could be an $18 billion disconnect between how investors are valuing BP's stock and what it's really worth.

Fool contributor Rich Smith does not own shares of any company named above. Rich is not a licensed economist, but he plays one on the Web. Check out his latest stock recommendations on Motley Fool CAPS. The Motley Fool has a disclosure policy.