Throwing them hither and yon to get a close look at pretty moons circling Saturn, Uranus and whatever is not. Many of those unmanned missions cost Billions.
They generally don't, actually. Mars Rovers were about 300 million a piece. The highest end-expense ones like Cassini do come in at 3 billion or so.
And they've actually discovered a lot of amazing stuff. Name me one interesting discovery of the International Space Station.
And the total costs of all countries involved for the ISS are difficult to estimate, but I've seen figures as high as 100 billion.