I'm not sure I agree. When optimizing for utility across one's lifespan, it's important to note that years of post-nuke life are both more expensive and carry less utility than years of non-nuke life. So when you evaluate the utility/dollar of building a fallout shelter and compare it to the utility/dollar of other potential investments, you need to put a discount factor on the years of life you expect your shelter to gain for you in the event of a war.

Um, utility tends to have diminishing returns in material possessions, hence the utility comparison goes the other way.