The 200 billion dollar may be high, but depends upon what the Manned Mars mission was. Considering that that space station was first estimated not to cost anywhere near 100 billion, the 200 billion number could be considered fairly accurate- perhaps even a bit low. I think an important consideration in what a Mars manned mission would accurately cost is the amount of time needed to develop. If it was fairly certain that NASA would be able to launch the first manned vehicle within 5 years of point of beginning the mission, then I think if you are only planning on a few trips to Mars then it's costs could be much less than 200 billion.
One major problem of planning on a Mars Direct is that at the moment there is no booster big enough. So, how do you get this booster?
At moment there is even some danger that we won't even have a Delta- IV H in the near future. The space launch market isn't too healthy now and it could become a problem is the future. If we do continue to have Delta- IV (and/or Altas V) but also have a fairly weak launch market, then it's quite possible that each launch will cost more than currently projected. In this environment building an even larger booster seems less likely- and if NASA is building it on it's own seems even more unlikely- then boosters could easily run as high as 5 billion each- say 10 boosters equalling 50 billion total program cost- if you wanted more boosters then the cost per booster could be lowered- by a lot.
In orbit assembly is of course possible, but there are number of problems with this- mainly it will probably end-up with NASA spending a lot of time building it, changing it, etc.
Then there is also the nuclear propulsion route. This shouldn't be considered merely as something good for Manned Mars. This technology needs to be developed- there will be uncertainties connected with this. In other words it's going to add more time to develop Manned Mars. Maybe the miltary will pay for this development (maybe not). Then you have the PR problem with Nukes.
Which brings us back to orginal point of Manned Mars- supposively going to Mars will have lots of public support [adding in Nukes to this equation? Sounds very weak to me.]
So if Manned Mars doesn't have much current public support- even though this idea has had fair amount of PR in the last 10 years. You have potential PR nuke nightmare. And without an increase to NASA funding, you ain't got the funds. So looks like years of spinning your wheels to me- could costs tens of billion without any progress towards a Manned Mars mission- sort of similar to current situation, only things could get much worse.

Now, let's compare to Manned Moon. First the boosters are already ready for going to the Moon- NASA should have ordered them a year ago- might have got a better price per launch. Still it's not too late- could be if NASA waits, say another 2 or 3 years before starting.
NASA could go to Moon and be done within 5 years of starting- total cost less than 5 billion. Don't really need public support, mainly Congressional. Increases NASA total budget by about 1 billion per year.
NASA does it right then funding for Manned mission more likely.
Now, suppose NASA does find minable water deposits at poles, and say some company (or several) wish to mine the water. This means more demand for hvy boosters. It also means you will have more business interest in Mars Manned mission (NASA is potential customer for lunar water). Etc.

Now, I will concede that NASA does have to do this Manned Moon the right way in order to succeed. And NASA track record in this regard isn't good. It has to be done in relatively short period of time in order that costs aren't spiraling out of control. This would definitely NOT be a lunar base.
Nor is NASA senting crew to the Moon in order to send crew to the Moon. They are only going to find water and find out if the water is something which could be mined. There isn't a requirement for this mission to be manned- it's just *likely* that this would be the cheapest way of really achieving the objective.
Of course, I've explained this all before. How it will increase national growth, potentially 100 trillion dollars in water at lunar, Space power- a trillion dollar market, etc.