While that's true, you have to launch the equipment to gather it and return it first. That still has a cost and currently they are prohibitly enormous.
Let me give you an example: 123m is what you get back in gold. Let's assume Falcon 9+ dragon can rendezvous with an asteroid(it can't). Falcon 9 costs 60 mil and some change to launch. That leaves 60 mil to manufacture a never before tested mining method that can remotely and autonomously extract and store ore. All of this has to weigh practically nothing due to the fact that once dragon is fully laden it will need to deorbit itself.
Performing a quick bit o math with a 4 ton dragon + 3 tons o gold...ahh we need to decide on an engine. The f9 second stage which can't operate beyond GEO, or dragon 2 hypergolic superdracos.
The first had a vacuum ISP of like 320 I think, the later is around 280.
Lastly, we need to know how much dV we need to deorbit. I'm going to assume our craft is on Earth escape which requires about(bit more) 3km/s of dv from Leo. For simplicity well stick with 3.
With that, you can solve for the amount of fuel needed to deorbit
DV = g*ISP*ln(m0/m1)
We need to solve for m0 - m1 to find the amount of propellant necessary. I'm on mobile so this really sucks.
ISP= 320 or 280
M0 = 7
So solve for m1 and subtract 7 and that's it. I'd wager it's a fair bit of propellent that isn't possible to store on the dragon alone, and that the second stage can't have in reserve(if it could make it that).I wanted to go through this math to show its not as simple as picking a vehicle and say let's try this. The physics of this are hard to begin with and economics make it even harder.
The point I'm trying to make is that using any current tech it's not economically feasible because not only do the craft to actually mine not exist, the cost of launching with current pricing isn't profitable. There's no way you can mine 3 tons of gold and return for less than 120 million, right now.
Not only that once you've launched something into orbit, it's more valuable staying there. Leaving the gravity well is the most expensive thing humanity can spend it's resources on. If Earth were a bit larger, the atmosphere a bit thicker, it would be nearly impossible to reach orbit without highly efficient engines(think nuclear or antimatter).
To expand on my original point in another post about the economics of a Martian colony, now you're leaving two gravity Wells. It's not going to be profitable. Peroid. Not for a long time because costs must drop.
This isn't a business venture it's a charity case to ensure the survival of humanity.
To reply to others, I read the responses yesterday but I remember agreeing with JB and thinking he has the right idea...whatever it was.
There's a reason this stuff hasn't been done. It's hard and expensive. Any other combination and it would have been done already.
Edit: reread the other posts, I pretty much reiterated with scattered thoughts what JB said.
Double edit: I realized I never summarized the AMA and I'll get to that tomorrow when I get home.