[Uehreka]

There are people in this thread saying tens of billions isn't that much in the long term (I'd agree) but there's a bigger point that comes into play whatever the price: The universe doesn't care if exploring it is expensive. You can't make a "that's not sustainable" argument to the universe and have it meet you half way. And that's who you're arguing against: not the scientists, the universe. The scientists don't decide how expensive future discoveries will be.