It’s not ridiculous. They estimate that large asteroids are just chock full of billions of dollars of rare metals that, if broken down and carried back to earth, it would pay for the trip itself. The machines to process the asteroids could be self-sufficient because the asteroids have enough water on them to fuel the machines and bring some back to earth.
It’s why Branson and Musk and others have taken a strong interest in space. There is potentially enough money to be made from this that somebody could become a trillionsire.
You are forgetting the cost of running preliminary missions over a 15 year period for the purpose of completing the environmental impact statements, then the litigation, and then the cost of reclamation and restoration to original contours.