The crazier thing is that for negligence torts (cars would be products liability, which is different), that's exactly how the law works.
It's called the "Hand Formula" and you are only liable for negligence when PL > B, where P is the probability of loss, L is the gravity of loss, and B is the cost of avoiding the risk.
In practice though, it can be hard to argue that sort of thing. It's a little academic.
Yes, of course. In a resource-constrained world, i.e. the world we live in, spending more on prevention than the expected loss makes no sense.
Refusing to spend $10 to avoid a $5 expected loss is not negligence, it's economic common sense.
(You should still be liable for the loss if someone else is experiencing the risk. You should be liable to compensate them fairly, but it's not negligence).
In fact in a world with scarce risk prevention resources, PL = B is not even a sufficient criteria to say you should spend to prevent the risk, because there might be opportunities to prevent greater losses elsewhere; i.e. you should not be investing to prevent PL = B type risks if there exists PL >>> B type risks. (For a finance analogy, if you are capital-constrained, you don't want to put your money in any investment with a positive ROI, you want to put you money in investments that have the highest ROI.)
In other words, spending $10 to prevent a $10 expected loss risk when there is an opportunity to prevent a $100 expected loss risk for the same expenditure makes no sense.
But since we're not smart enough to cooperate and globally allocate the total risk-prevention budget to minimize losses, locally and in terms of personal responsibility PL = B is a decent rule.
It's called the "Hand Formula" and you are only liable for negligence when PL > B, where P is the probability of loss, L is the gravity of loss, and B is the cost of avoiding the risk.
In practice though, it can be hard to argue that sort of thing. It's a little academic.