Are the people arguing against rlumley actually arguing that cost/benefit is not issue or that rlumley's got his cost/benefit estimate wrong? The two are vastly different.
Seatbelts are very cheap and have a demonstrable safety effect. Their cost/benefit ratio is very low. Preventing conventional wars through strong national defense arguably has a low cost/benefit ratio because the casualties caused by conventional warfare are massive. Most countries seem to achieve peace without nearly as massive defense budgets though. Airlines outfit their planes with appropriate precautions based on cost/benefit analysis. If you think planes are absolutely as safe as they can possibly be, think again, they're as safe as is reasonable in terms of cost. You can't spend infinitely on safety... think of what you'd have to give up to do so.
Anti-terrorism spending is massive compared to the lives saved. Ok. I think we all agree that there have been less terrorist attacks on American soil because of Homeland Security. It's hard to know how many have actually been prevented, or would have occurred without Homeland Security, but let's be reasonable. How many lives could have possibly been lost if Homeland Security had half its budget. What about a quarter? How many more lives will be saved if you double Home Sec.'s budget? I'm inclined to believe diminishing returns are in play here. You have to weigh the ratio to determine whether or not it's worth it. Octavious brings up a good point regarding the money lost in stock markets due to terrorist attacks. That should definitely weigh into the cost/benefit ratio. You also have to consider the opportunity cost of the money spent. If it costs a $5000 in police expenses to prevent/deter a homicide, or $14000 to give an American the medication they need to survive, or $20000 to properly execute a heart transplant, and $1 million dollars to prevent a death related to a terrorist attack... I've made up the numbers completely, but the point is that it's a matter of the numbers.
On a separate point, airport security is absolutely daft. You can't bring shampoo but you can bring a lighter on the plane. *cough* Tobacco industry lobby *cough*. Oh, but you can actually bring a lot of liquids as long as they are in small individual containers. Couple of ounces of plastic explosives in my toothpaste, another couple in my shampoo, another couple in my deodorant. Airport security is practically a symbolic gesture. The actual measures are absurd.