Your belief that a lunar observatory is the cheapest option indicates your total lack of engineering expertise and a shortage of common sense as well. A Robotic observatory to detect 100 m objects quickly would probably cost on the order of 100's of millions of dollars. A ground based scope which could go down to 300 m (in ten years) for a few tens of millions. Such a scope could detect most 100 m objects in less than a century a represents the low cost but slow option. The speed however is around the expected interval between such impacts and therefore not a grossly negligent approach.

I think spending ten times as much to reduce the odds of surprise by a similar order of magnitude is justifiable especially given other potential cocurrent uses for a space based NEO survey scope. Spending another ten times as much given for no significant increase in capability is just a waste of taxpayers money. These rough cost estimates are based on comparisons to other projects of similar scope in already in the planning stages or already completed. If I'm off base I'd happy to be corrected by Larry J or others with actual space experience. It is ludicrous in my view for you to claim a man tended observatory could be done for less than 1 % of the inflation adjusted cost of Apollo.

Regards,
Mark