100 million dollars can go a long way to feeding people that don't have food. a 100 million dollars can go a long way to rebuild america's crumbling infrastructure.
$100 million dollars spent towards solving humanity's greatest mystery is absolutely nothing compared to what we spend on building a single fighter jet. Looking at the big picture, this is a steal.
Plus, if you can't justify to yourself why finding extraterrestrial life is important, then you shouldn't be talking about science to begin with.
Why not just have people do it because they're passionate about it?
Science is loving expensive. 'Passion' is great, but whoring one's self out for money is also important. It's a necessary evil.