One of the things I always like to point out in the "Manned versus Unmanned" arguments is comparing the amount of lunar material brought back. The Apollo program returned something like 800 KG of moon rocks. The Soviet Union's landers returned something like 0.8 Grams of moon dust. And those rocks were brought back because an astronaut (who in later missions was trained in geology) actually thought they were interesting, whereas the moon dust returned by the Luna probes was whatever happened to be within reach.
So it costs a lot more. The question is, do you get more value out of a manned mission versus a robotic mission? Apollo brought back 1,000,000x the amount of lunar material for 1000x the cost. So if you're just calculating based on those numbers, Apollo gave a better return than the Luna program. But that initial cost was pretty off-putting.
As people at NASA and others have pointed out, what the rovers have accomplished on Mars could have been done by an astronaut in a couple of days.
An analogous issue is time versus money. I could buy a ticket on the Concorde way back when and get from New York to London in three hours for $6000. I could buy a ticket nowadays on a non-supersonic transport for maybe $1500 that would get me there in 8 hours. The question is, is it worth the extra $4500 to get there five hours earlier?
Mars isn't really changing. There's really nothing about Mars we need to know right now, such that it would be worth spending that initially large amount of money to find out.