Actually you should look at the value of the target to be hit by the missile, not at the value of the missile to be intercepted. You can destroy infrastracture and equipment worth of millions of dollars with a single missile, not to mention that most of the stuff that is destroyed can't be replaced instantly. The $100,000 cost of the interceptor is small compared to that.

Around here, there is a notable difference in quality of the road surface between the lanes, because there are plenty of heavy load trucks. I actually do drive in the passing lane on an empty highway just to save on shocks maintenance costs.