The Red Planet is surrounded by the rubble of failed worlds — rocky boulders stuffed with metal. Shipping a kilogram of steel from Earth to Mars costs as much as several cars. It's far cheaper to pick up space junk right in orbit. Our Galaxy is full of rocky debris rich in iron and nickel — ideal raw material for building a colony.
Some of this debris contains carbon — from it, you can produce hydrogen fuel (light rocket propellant). So the ship's return trip is paid for by the asteroid itself. Trajectories are calculated to spend the least energy on acceleration and braking. The imported metal will go straight into 3D printers that churn out living domes, rovers, and greenhouse frames.
Amazing fact: a single medium-sized metallic asteroid weighs more than all the skyscrapers in Manhattan, and contains more iron than all of humanity has ever mined. Space junk becomes the foundation of Martian cities.
Most metallic asteroids are fragments of ancient planets that never managed to coalesce into a single body.
In the TV series 'The Expanse', the fight for asteroid belt resources is the central conflict between Earth, Mars, and station dwellers.