The most valuable substance in space rn is water. strip hydrogen and oxygen using solar power and place it in orbit. Instead of launching a Mars-bound rocket with 90% of its weight tied up in fuel, you launch it nearly empty, fly to an orbital fuel depot, fill up on asteroid-mined propellant, and go. Big business.
Or maybe you process insitu, nstead of a 30-million-ton rock, you return a compact, high-density capsule of pure platinum. A few tons of pure platinum group metals dropped into a stable Earth trajectory fits comfortably within existing down-mass logistics and could be worth billions.
Thirdly, (prolly the worst of all the options), you place metal slag in orbit and you drop it on peoples heads. Now you've got rods of god that any military will pay billions for.
Edit: Lastly, once you control a vast amount of stratigically important material they would instantly become a sovereign-level geopolitical superpower. The moment a private entity controls a constellation capable of dropping kinetic rods anywhere on Earth within 15 minutes, every terrestrial military becomes fucked overnight. The line between industrial infrastructure and planetary hegemony completely disappears. ya gotta price that in somehow. Then... we really have realised the vision laid out in the book "the soverign individual"
Edit 2: >where is the Madre De Deus of this ISRU demonstration
If everyone could do it, they would have done it. The Madre De Deus was an absolute marvel of engineering at the time. the english knew the portuguese were selling all the stuff from the colonies, but they didn't really know how freakin' profitable it actually was. The Dues was freakin' massive. AKA Starship.