"Larry [Page] asked me a couple weeks ago how much it would cost to send people one way to Mars and I told him $10 billion, and his response was, ‘Can you get it down to 1 or 2 billion?’ So now we’re starting to get a little argument over the price.”
Larry is currently listed on Forbes as being worth $14b, so $1b really isn’t a huge amount of his total cashpile.
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.