Comment by JoshuaDavid

Unironically, it does seem worth spending the entire GDP of Hawaii on a 1 / 6 chance of intercepting an ICBM, if there's a nontrivial chance that you're likely to have a small number of ICBMs incoming towards cities. Other than second-order effects where anything which breaks MAD increases the risk of nuclear war, but I hope a 1/6 chance of interception of one missile wouldn't be significant enough to change the balance of power enough to make nuclear war more likely.