you don't have to ship things the moon, you just build a mass driver on the moon that sends things to earth. it doesn't need to yield diamonds, this would be lucrative with just fresh water
Shipping on water has been, by far, the cheapest mode of long-distance shipping since the moment boats were invented. That is to say, since thousands of years before boats were ever powered by the shit that destroys our lungs.
It is valuable if they can find the right rocks and bring them back. A platinum group metal asteroid would be of immense value, at least the first one anyways. After that who knows, they might super saturate the global market for decades.
our use of platinum has been limited by its scarcity, having tons of it would completely change the things we could build. saturation isn't a real downstream effect of economics, it would instead be transformative