The idea of humanity moving to Mars is nothing new. But what would it cost to build an entire city on Mars? Well SpaceX CEO Elon Musk has estimated that number to be around $100 billion to $10 trillion. Musk revealed this in a tweet while replying to Marstronauts.

Musk tweeted his estimates of building a city on Mars in response to a question posed by the Twitter handle @marstronauts.

My question is, what’s your current estimate on the total cost of having a self-sustaining civilization on Mars?

— Marstronauts (@marstronauts) August 9, 2019

According to the estimate by Musk, building a city on Mars could cost anywhere between 10 per cent of the US’ 2019 military budget and three times the the country’s 2018 tax revenue.

Musk also calculated the approximate future cost of sending a minimum payload to Mars “to nearest order of magnitude”, at $100,000 per tonne.

So if building a self-sustaining city on Mars requires a million tonnes of cargo, the cost would be around $100 billion, he calculated.

Musk has been vocal about his plans for colonising Mars over the next 100 years as according to him, the Earth will face a near-extinction event at some point in time and thus the need of a “backup” plan for all humankind.