Space flight has military benefits ("Hey look - we can go into space too, so don't fuck with us!"), scientific benefits (experimentation in a zero-gravity vacuum, material analysis of the interplanetary medium, vastly improved telescopy), immense practical benefits (without rocketry we wouldn't have cell phones and GPS devices, for example), and potential long-term benefits (the possibility of space colonies, protecting humanity from extinction due to a variety of existential risks). Since NASA is actually fairly cheap as U.S. government projects go, and since spaceflight is increasingly moving toward becoming a private endeavour (made possible, in part, by decades of publicly-funded spaceflight) you'd be hard-pressed to demonstrate that the cost-benefit analysis comes out in the negative.

EDIT: That's also not including the quality-of-life benefits that come from getting to watch something this totally awesome.