This isn't a viable experiment for ICBM interception due partly to the size of the target. That's why we have missle interception tests over the pacific (which I totally support). As far as comets or astroids hitting the earth, I'll put my money on the good Lord, not NASA.
Not all high-speed interceptions are always going to be missiles; some may very well be of comets or asteroids on Earth Impact Trajectories, for instance...
...And the Good Lord helps those who helps themselves...and he's also warned us by showing us that such celestial objects have hit the earth in the past, with large losses of life.
I'd also add that your *fetish* with not spending money is a wasted talent. Should the Earth be hit by an Extinction Level Event due to *not* spending enough money on earleir interception tests, the padded bank accounts from said cost savings will have utterly no value to a dead Race of Man.
Tunguska and the various large-scale impact craters on earth suggest that faith, admirable in it's own right, might benefit from a bit of technical assistance