Manned vs unmanned

I'm not sure if this is the right news group to help with this question.
I had an argument with a friend the other day about whether its cheaper
to send a probe to mars than it is to send humans. I say its cheaper
by a long shot to sent probes (ie robots) rather than astronauts to do
space missions. He feels that its the opposite and that NASA is
reluctant to send anyone to Mars for fear of failure and killing humans
(i.e. a public relations disaster). So we figured we need to specify a
particular mission to help narrow things down.

Mission:
Lets say NASA wants to get some rock samples from Mars to bring back to
Earth and that the rocks will be collected from an area 1 km by
1 km.

Scenario 1 - Probe/robot: Lets say the cost for sending the probe not
only involves those associated with the actually transportation and
space craft design, but also the cost of developing, manufacturing and
maybe possibly requiring two missions if the first fails. There may
be some other costs I might be forgetting though.

Scenario 2 - Human: Other than `standard' training and salary for the
astronaut (which I imagine is minimal wrt to all other costs), space
craft design, life support. As well, there could be other costs I'm
forgetting.

Question: which is more economical in dollars, scenario 1 or scenario 2?