The rocket payload doesn’t really apply here. You’re not putting something in space, your bringing it back. In theory, you can just drop part of a raw asteroid into Earth’s gravity and let it fall in some unpopulated safe zone. It doesn’t need an expensive reentry vehicle. Of course the issue is safety, it’s probably unacceptable to just drop it. Maybe it needs some minimal reentry vehicle to guide the asteroid fragments to the proper landing zone. That should be much easier/cheaper than getting living people down safely to a launch pad.
I wouldn’t think you would be doing this with iron, and I doubt you would find a large enough block of high purity gold or platinum to send down, but there may be options in between. The point is, the refining doesn’t need to happen in space, all you need is a way to cut apart the asteroids. And of course you need a way to detect which ones have more valuable material content. Bonus points for doing this using an automated system so that the mining ship doesn’t require a crew.