Let's see your numbers!
I mean, imagine how much it would cost to build just a blast furnace on Mars. You're going to need a blast furnace, because one of the few raw materials we know exists on Mars is iron.
So where do you get the power for this? Solar? Mars is not so well suited for solar power unfortunately - it's twice as far away from the Sun, but worse, dust storms are common and seem to be extremely destructive.
Also, you can't make the solar power cells on Mars, until you already have heavy industry on Mars, so each one has to be brought to Mars.
There seems to be no obvious solution, particularly since each Martian will use two order of magnitude more power than your average American.
This article goes into detail: https://medium.com/predict/mars-big-problem-does-elon-know-7453dcb5feaf
Where do you get the carbon to make steel? Where do you get the 20,500 gallons of water you need to make a ton of pig iron? Where do you get the oxygen?
How do you construct it? With the native materials? But Martian sand is not a good building material. More, it's filled with poisonous perchlorates, so if humans were in contact with it, you'd need to somehow clean the sand before you used it.
Where do you mine the ore? Are you going to get it from the sands? That's going to take an order of magnitude more energy. You're going to have to start mining anyway for rare earths, aluminum, cobalt, copper, you name it.
How do you mine on Mars? Are you going to have people in spacesuits with pickaxes? Are you could construct an army of drones to operate that mine and ship them to Mars? We're decades away from any technology like this that would work on the extremely pleasant environment that is the Earth.
Meanwhile, while you're building that blast furnace, you need to provide food, water, oxygen, heating, places to sleep, medical care, and all this for all your workers, and every gram of that has to be tracked 300 million kilometers from Earth.
---
About twenty years ago, I became curious as to why there were absolutely no detailed plans about what would need to be done to set up a Mars colony.
Sketching it out made it clear why - there were tens of thousands of separate facilities that needed to be constructed, things that would each cost millions of dollars each on Earth, and far, far more when they had to be made completely airtight and then flown to Mars.
More, we simply have no idea how the key parts would work, or if they are even possible.
How exactly would we "make air"? From what? The water we know is there in tiny quantities? But we need that to drink, to grow food, and really for almost every form of manufacture known to man.
"Oh, we'll invent new techniques," but we'd have to re-invent pretty well every piece of manufacturing technology we have ever created to do this.
There isn't even a sketch of a full plan, because the sketch would make it completely clear how incredibly expensive it would be.
And we have no idea if humans would thrive. Would people really enjoy living in pressurized domes for the rest of their natural lives, and their childrens', and their childrens'?
---
And for what? A planet where you can't ever go outside without a pressure suit? A planet that seems to be poor in raw materials - and even if it wasn't the economics of "shipping" from Mars to Earth are totally inconceivable?
Terraforming Mars would take hundreds if not thousands of years - and even then, the surface temperature would make it unlivable without protective gear except for a few weeks a year near the equator (and very far from any sources of water).
And in order to terraform Mars, Martian engineers would have to perform engineering feats a hundred times greater than any such feat so far on Earth - but every year, for a century, for no benefit to themselves.
---
Meanwhile, our planet is in danger, now, this instant. In my lifetime we've killed over half the wild mammals, well over half the aquatic mammals and most of the aquatic creatures bigger than my two fists that aren't jellyfish, about half the flying insects.
Those little green dots in this picture are all that are left of the wild mammals: https://xkcd.com/1338/ Even a hundred years ago, that picture would be the reverse.
The last time the CO2 levels were this high, water levels were 60 meters higher than today. Climate disaster is already baked into the system - the question is whether we can avoid a catastrophe.
Against the background of this immediate problem of saving our biosphere, a very expensive problem to fix that we aren't addressing anywhere near adequately, to propose spending our limited resources on a project that is orders of magnitude more expensive and won't help your average human one bit shows a complete lack of a sense of proportion.
Imagine I was about to be evicted from my house for not paying the mortgage. Instead of dealing with the issue, I am are trying to use my money to put a downpayment on this hotel https://en.wikipedia.org/wiki/Ryugyong_Hotel
You'd think I was crazy. But this is not as crazy as letting the Earth die in favor of Mars.
