I am opposed to spending 30-40 billion dollars to get a man to Mars and back at this time. Now, my saying this does not mean that I am opposed to finding out more about Mars. Out of all our planets, Mars is one that shows some of the greatest potential for life in case the Earth does not last, not to mention it is one of the closest to get to. I do think, however, that before we consider spending such a sum, we need to first get more information about Mars, preferably through probes and other space vehicles like rovers, so we will have the best chance of moving humans to Mars in the safest way possible. Also, when we have so many other priorities right here on Earth to consider, like people who can’t feed their own families and pay their bills, I think that money is best spent on Earth-related issues.
We have had 6 successful probes sent to Mars so far, but there is a lot more involved with sending a person to a planet rather than a probe. We have to consider the overall weight, not only for fuel, but for provisions and the astronauts themselves. If we miscalculate, the astronauts could burn up, go right past the planet instead of into its orbit, or not be able to generate enough velocity to get OUT of Mars’ orbit. Yes, we have learned a lot about Mars’ atmosphere, but we need to know more and perform “practice” missions to be sure we can account for most circumstances.
Several well-respected men have had the same thoughts I have had in terms of prioritizing other issues before space travel. For instance, Martin Luther King stated, "If our nation can spend twenty billion dollars to put a man on the moon, it can spend billions of dollars to put God's children on their own two feet right here on earth." Also, Joseph Lieberman indicated that the money would be needed "right here on Earth to give health care that's affordable to everybody, to improve our education system, and do better on veterans' benefits and homeland security". Even Howard Dean thought that "space exploration is terrific", but wanted to know "where is the tax increase to pay for it? It is not worth bankrupting the country."
I would much rather spend the better portion of this money on making the lives of Earth’s population more rewarding, and a smaller amount of money getting more information and making well thought out, careful decisions about what will be needed to successfully travel to Mars in the future.

References:
Herath, A. K. (2011, April 18). Why Is It So Hard to Travel to Mars? | Future NASA Space Exploration & Red Planet. Retrieved from http://www.space.com/11417-mars-missions-space-travel-challenges.html
Pop, V. (2004, January 19). Is Space Exploration Worth the Cost? Retrieved from http://www.spacedaily.com/news/oped-04b.html