I think, if we want to play 'what-about', then it's fair to say that the cost of the New Horizons mission ("approximately $700 million over 15 years", per Wikipedia) is a drop in the ocean compared to the US Dept of Defence's direct spending on the 2003-2010 Iraq war, which was more than $700 billion, which came just as much out of the wallet of the US taxpayer (who is both rich and poor, and separately, both content and suffering), and as a little-known side-effect, killed a bunch more people than New Horizons has so far.