Astronomy is pretty well understood, so it is pretty easy to estimate the cost of searching the sky for dangerous objects.

Sort of. The possibility of mirror matter objects makes this pretty difficult. There's even a reasonable-if-implausible paper arguing that a mirror object caused the Tunguska event, and many other allegedly anomalous impacts over the last century. There's a lot of astronomical reasons to take this idea seriously, e.g. IIRC three times too many moon craters.

Reality check: mirror matter has a gravitational signature - so we know some 99% of non-stellar matter in the solar system is not mirror matter - or we would see its grav-sig. So: we can ignore it with only a minor error.