I am not expert on this and I have not done the math but the amount of supplies needed to last two people for forty years in a hostile environment would be more then I think you could fit into the payload of a single rocket. They are going to need to make air, find and purify water, build permanent structures, grow food ect. No matter how well you plan they are going to need to need a constant stream of new supplies. They will need filters to scrub the air, new batteries, medicine parts ect. Sending them there with no way to get them back is a life time commitment to keep them supplied even after people have grown bored watching them try and stay alive. Look at how Bio-dome II failed. We will need to do better than that if we expect them to live.

If you are going to plan a round trip to mars you will need to figure out how to make rocket fuel on the surface of the planet so that really on cost you the plant to make it and what it cost to get the plant there.

One of the reasons it is much cheaper to send a robot is it doesn't need the supplies a human does and when it breaks down you can ignore it.

Short term I think it makes more sense to use the moon to learn to support a station in a hostile environment. If something goes wrong you can be home in three days and new supplies could get there in about a week. A permanent moon base would teach us a lot about building one on mars and would make a mars mission much more likely to succeed. If you are not going to do that then the next best step is a temporary space station above mars that would give you real time control of robots and some experience traveling that far in space. Mars is much harder to do then the moon.

The idea that we need to colonize mars because we are killing the earth is simply stupid. I can’t imagine that how we could damage the earth enough to make it less hospitable then mars. Besides a self sustaining mars colony is hundreds of years away at best, it may never happen.