That was a very dumb thing for Sagan to say. It is very rare for anyone to be interested in any question about which he has absolutely no evidence. The reason that flipping coins is a thing is that humans intentionally searched for ways to artificially create the very rare situation in which a person genuinely does not have any evidence that would favor one outcome over another. (Human performed this search because humans prefer to play games where both participants have a decent chance of winning).

The complete lack of any signs of any alien civilization capable of making UFOs in the tremendous volume of astronomical, geological and geographical observations is evidence that tilts the probability far away from 50-50.