>concentrating one gram of platinum from one ton of iron would be tremendously difficult, requiring a huge amount of energy.

melting one ton of iron requires 500KWh, 12 gallons of gasoline, less than $100 on Earth. Or 5 Tesla car batteries fully charged by say 30x30 m solar array in 2.5 hours - cost nothing in space once you got the hardware there. This is why mining in space is going to be a pretty big thing once/if we get cheap launch capability.