Well, it's not like you are just going to grab the whole thing and bring it back. But I agree, a new source of rare metals would change the pricing. But, platinum is an interesting metal -- engineers could do a lot fun things with it if it wasn't so expensive.

In one asteroid is worth $20 trillion, how much are 5000 asteroids worth?

I expect if you had 5000 asteroids worth of iron, you'd have to pay people to take it off your hands.

An 800m iron asteroid would make a rather nasty weapon.... (about 75000Mt, probably considerably more than all of the nuclear weapons that have ever existed).