"It will probably vary depending on the type of mission, but I would imagine (on the average), that for the cost of one manned mission you could send ten different robot probes with ten separate missions to the same destination. For the cost of one extended colony on another world, you could fund a complete robot space program visiting many worlds, even making repeat visits if they turned up something really interesting."

Google answer:
"The cost of the Apollo space program is commonly given as $25 billion.
When adjusted to 2005 dollars this would approximate to $135 billion."
http://answers.google.com/answers/threadview?id=601086
And:
"The cost of the entire Apollo program: USD $25.4 billion- 1969 Dollars ($135-billion in 2005 Dollars). [...] Apollo spacecraft and Saturn rocket cost alone, was about $83-billion 2005 Dollars (Apollo spacecraft cost $28-billion, Saturn I, IB, V costs about $46-billion 2005 dollars)."

So, the part of program that built the Saturn V cost about 1/2 the entire cost. The Saturn V was and is the biggest lifting rocket ever make. Mars fans would cut off their right arm to have a Saturn V type rocket. A Saturn V lifted about 5 times any other rocket that has ever existed- though the Shuttle is about 1/2 it's lift- a Shuttle C [a non existent rocket variant, could lift about 1/2 the payload].

Now, why did NASA build the Saturn V and then end the Saturn V? Much could said about this and much has been said about this. But the simple answer was building the Saturn V was cheaper- assuming you want to get crew to the Moon before the end of the decade, and safely return them. It was cheaper only in the sense of the task one needed to be done- meet the goal set by a US president that was shot dead.

Counting the cost of anything is not easy. For instance, in terms manned programs, you had the Mercury Program, the Gemini Program and the Apollo program. The spacecraft for any of these would been the main cost of the program.
The other big chunk of the cost is all the people required to do any of these government programs and this cost is of course interweaved into the spacecraft costs, but we know how people are employed by NASA and we could figure of there costs- wages, healthcare, retirement, etc.

I'm not going to do all that work, but NASA has about 18,000 workers and roughly each costs about 100,000 per year- though it's quite possible that it's likely it amounts to about 200,000 per year. So 18,000 times 100,000 is 1.8 billion per year- that's the cost for them to show up at the job- regardless of what they actually do.
And this is why the Shuttle cost more than 1/2 billion dollars per launch- all the people employed in the shuttle program plus the costs of buying hardware and rocket fuel [though just the costs of the rocket fuel utterly insignificant, relatively]. And this total cost is divided by the number of shuttle launches per year- the more shuttle launches per year the cheaper the unit costs, though of course each launch cost has what called the marginal costs- need to pay for the solid boosters, External tank, and repairing the orbiter so ready for next launch. The marginal cost are about 1/3 of the cost of a launch, or said differently the NASA workforce is about 2/3rd of the cost of each launch.

So, very roughly, the job program part of NASA cost at least about 2 billion dollars per year, 20 billion per decade, and about 100 billion since NASA started. I'm sure it's more that. But that's to have them show up, regardless of what they do. And then you have all the other people working not as government workers, but involved in some way- whether it building office spaces, or cleaning office spaces, or whatever. It's like a army- you have the people who are directly involved in the fighting and you have all the logistics or the tail of the army. It's the logistics which is the important cost of a army and determine it's success more than simple combat tactics.

Robot vs Manned is combat tactics.