I've seen numerous proposals costing far less than a lunar based one. The Moon has some big disadvantages including the biggest of obstructing half the sky compared to an orbital scope. We even have ground based proposals for taking care of most of the midlevel threat in relatively short order like PANN-Stars, LSST, and DCT.

An actual protection system may be another matter, but an expensive to maintain system likely to go unused for decades is unlikely to sustain long term funding without demonstrable need. Humans have a tendency to ignore or disdain theoretical threats until realized. Just like the recent tsunami finally pushing India to fund a warning system. We might not even have the program we do if not for shoemaker-Levy 9 and finding Chixalube. I firmly believe that a sustained human presence in space is our best long term protection by making space travel and infrastructure routine enough to be readily diverted to defense if needed.

Regards,
Mark