A post at io9 evaluates the cost of robotic interstellar probes:
When does it make sense to build a starship? Back in the late 1960s, Freeman Dyson went to work on the question of how much an interstellar probe might cost. Extrapolating from nuclear pulse propulsion and the state of the art in spacecraft design as then understood, Dyson arrived at an estimate of $100 billion to build the craft, which translates into roughly $650 billion today. Though stark, that figure is by no means as eye-popping as one of the estimates drawn up by the original Project Daedalus team: $100 trillion in 1978 dollars. ...
[Richard] Obousy's assumption is that it will become economically feasible (and just as important, politically viable) to construct an interstellar craft when the total cost represents no more than 1% of the Gross Domestic Product of the constructing nation. Now the GDP of the United States, a measure of the country's overall economic output in a given year, is currently $14.6 trillion. The GDP of the entire planet is now $61.1 trillion, and the option of consolidating the larger amount in these calculations is obvious, so Obousy looks at his figures in terms of the US economy and contrasts those results with the global perspective.
The outcome: The US economy becomes able to support a Dyson-class starship costing $650 billion by the year 2085; i.e., in that year, such a cost represents 1% of GDP based on a 2% growth rate per year. A Daedalus-style craft becomes feasible no earlier than 2340. In global terms, the Dyson starship could be built (assuming the global cooperation we at present do not have) within the next few years, whereas the Daedalus class craft would have to wait until 2268.
But what are the benefits? Just because we could afford it, doesn't mean we ought to do it.
Obviously, there will be some benefits from development of new technologies. The infrastructure to support interstellar probes likely could be used to support economic exploitation of our solar system, as well. The biggest benefit, however, would be diversification by diaspora. As long as the human race is confined to one solar system--let alone one planet--we are vulnerable to mass extinction events. The goal of space exploration, it seems to me, is to enable humanity to stop putting all its eggs in one basket.
If I'm right about that, it has implications for current spending. Put simply, every penny of NASA's budget ought to go into reducing the cost of a Dyson-class starship. Sadly, however, I suspect that's way too much in the way of long-range thinking for our government.