By better understanding the far reaches of the universe, we attain a better understanding of ourselves. Being able to see further allows us to study things we can not study up close. Many breakthroughs in physics, which have resulted in the inventions of new technology came from discoveries made possible by deep space exploration.
Second, if you truly believe that a telescope serves no better means than just seeing further, you are mistaken. While that is one perk, it also is frequently forgotten that a better telescope also allows us to see in better detail the universe that isn't so far away. It is a fact that there will come a time where humankind will have no choice but to migrate away from Earth, or go extinct. We unfortunately can not predict when this will happen. The right natural disaster could force us to do so within the next 100 years for all we know.
Finally, if we are ever able to prove that another sentient life-form exists out there somewhere, it will have a huge impact on humankind because we will finally know for certain that we are not alone. Imagine being raised in the wilderness by one person, never having met another person or even having any knowledge that other people like you even exist. When you eventually made the discovery of more people similar to you are out there - your mind would be blown. We as individuals may not feel the loneliness on such a grand scale - but the human race as a whole does have a "psyche" of it's own. We share our experiences on Earth in one way or another. That great loneliness being lifted could put us into a new age of enlightenment.
Besides, with 300 million people in our country, assuming 2/3 of them pay taxes - that's only a few bucks out of our pocket. We spend more than that to watch a silly movie at the theater - and that $700 mil is a one time cost, so it isn't like that will come out of our pockets month after month.
Quite frankly, I think it's a much better investment than to hand out 100 times that much to the banks that have screwed us over so bad we won't fully recover for several more years...just so they can buy bigger yachts.