Does that calculation change if the goal is not to bring minerals back to earth, but to mine them and use them in situ to build habitats and the like? I imagine a bunch of sand and iron and ice would be quite useful in those circumstances
I do agree the idea of bringing them back to Earth makes no sense, and the technology necessary to do that will probably make it not necessary thru other breakthroughs. But if you believe that Earth isn't the only location for humans to live we won't want to bring ML (Mega Liters) of water to future habitats, we want to either find it, or manufacture it.
generational ships could spend entire decades in pockets with no sun...
all you need is the right raw materials, access to water and food, and fuel source to keep the uv lights working and the heater and oxygen pumping...