One of the things that make it tough to figure out how much has to be charged for insurance is that people behave differently when they are insured from the way they behave when they are not insured.
In other words, if one person out of 10,000 has his car set on fire, and it costs an average of $10,000 to restore the car to its previous condition, then it might seem as if charging one dollar to all 10,000 people would be enough to cover the cost of paying $10,000 to the one person whose car will need to be repaired. But the joker in this deal is that people whose cars are insured may not be as cautious as other people are about what kinds of neighborhoods they park their car in.
The same principle applies to government policies. When taxpayer-subsidized government insurance policies protect people against flood damage, more people are willing to live in places where there are greater dangers of flooding. Often these are luxury beachfront homes with great views of the ocean. So what if they suffer flood damage once every decade or so, if Uncle Sam is picking up the tab for restoring everything?