You'll have to add much more weight just in auxillery systems. Like the food and air issue. We can't rely on the prospect of using mars materials too much because it means doom if those systems fail. So I imagine they'll need to take more food and air than those basic minimums.
I think Daver was dead on about a long term committment and the current lack thereof. NASA gets new priorities with every new administration, even every fiscal year.
I'm not yet convinced a manned mission to Mars is going to produce the scientific results that probes will, and we can launch a lot more probes for the cost of a single manned mission- alot quicker too.
Although Apollo was a great achievement, was it really the most efficient way to explore the moon? Not really. I doubt we'll ever see the day where NASA gets such a huge budget or congressional support again, so bang for the buck is more important than footprints in the sand.
I do disagree with China having the foresight to go to Mars, or even the moon for that matter. China is driven by military paranoia at the executive level. Space is a military platform for them, and little else.