[Nokinside]

Billions. It might cost 20 billion per year (the whole NASA budget) or several times of that.

Economically speaking Mars would not achieve economies of scale needed for becoming self sustaining.

Manufacturing every component for higher safety standards than commercial aircraft or nuclear submarine is expensive. Building whole society that works in safety critical environment 24/7, starting from daycare and ending with microprocessor and chemical plants is just fantasy in the near future.

Maybe in 100 or 150 years we can start dreaming.