But it was a one-way trip... I assume a trip there and back would be a tad bit more (2-5X).
IMHO designing a system that would enable a pretty good odds of coming back would be at least an order of magnitude more expensive than a similarly reliable one way system.
Think of all the additional R&D and engineering difficulties involved in keeping a human alive long enough to get there, do something there come back and survive.
http://en.wikipedia.org/wiki/Sabatier_reaction
And you get oxygen to breathe for free! The Mars Direct plan calls for the initial deployment of an unmanned mission to set up a fuel station on the surface - then unmanned missions to cache supplies there - and only when you have enough stuff pre-deployed for the return trip and/or a permanent presence, send humans.
The moon may be closer, but until you have a working fusion reactor, there's not much you can usefully do there in terms of resource extraction.
Each launch enables a one-way trip to or from Mars.