> 100kg of that stuff my be worth 3 - 5 million $

That might not be enough to break even.

The asymptotics work out much better with Asteroid mining.

Curiosity cost $2.5 billion. Round it up to 3 since it's a round trip. You'll have to bring back 200,000 pounds of platinum to break even. That's like 3 or 4 Sherman tanks.

It will get cheaper as we get more reusable launch vehicles!

Resource prices will also drop in response to a larger supply. That's what I was taught once upon a time. Now of course you have the likes of Goldman Sachs manipulating prices by stockpiling resources to lower supply.

I was imagining that your typical nickel-iron meteorite would have a couple of percent of interesting impurities. I couldn't find any really good sources of information, but it sounds like -- with the exception of Cobalt -- any other recoverable metals would be in the parts per million, so not worth the trouble.

On the other hand my plan to cast meteoritic iron into shells and shoot them to Earth might still be practical, if only there was something suitably valuable to put into them...

Just be careful you don't accidentally turn up with 100 KT.

Price is a function of demand ... and supply.