While this may be true on long timescales, in the short term I think almost everybody would agree that the money would be better spent on raising the quality of life for people here on Earth rather than manned space exploration and colonization. There is a time and a place to begin to begin wading out into the cosmic pond, as it were, for survival's sake, and we clearly are not at that point yet. So the only other reason then for manned space exploration besides survival is science, and Chronos is absolutely correct -- humans are marvelously inefficient at this. The benefits involved in sending humans are completely outstripped by the extra costs.
For example, while it is true that a mars rover might make a poor decision and get itself lost, stuck, or miss something we might consider "interesting", for the cost of sending a manned mission we could have dozens!
It's a glorious idea, sending humans to space and all, and it involves a little bit of wonder in us all no doubt. But simply because it strikes a chord with us does not make it practical