There's a great little article in this week's Space Review by David Livingston, host of The Space Show. The question was, "is manned space exploration worth the cost?" and you can read the article yourself for the background on exactly how this came up.
Livingston makes several points, the first being one that I have often noted, which is the rather obvious fact that money spent "on space" is in fact spent right here, mostly in the US, and mostly to employ Americans of many occupations, not only scientists, engineers, and astronauts. He also notes that there are various types of "government programs," some of which only cover immediate expenses of various sorts with no long-term benefits, and others (like the space program) that contribute to education, industrial growth, spin-off technologies, and new business development. Space has also been an area of peaceful international cooperation that has emphasized and supported interests shared by many countries, including countries that have not tended to be all that friendly otherwise (think the USA and the former USSR in the seventies and eighties).
Finally he notes that for a well-run manned space program (for which he uses Apollo as the prime example), there are important "human spin-offs." I am one of many engineers and scientists who chose to pursue a technical education and career in large part because of the inspiration of the Mercury, Gemini, and Apollo programs that took place when I was growing up in the sixties. This is a legacy unmatched by any other government program you can name.
Space exploration (human and robotic) is definitely worth what we spend on it, and on many different levels. It's my favorite 0.6% of the federal budget and I wish it could be more!