From what I read of Sempra, they are brokers in copper, etc.
Therefore, I think CPRK went to them as an entry-level point in the market. There are other brokers, but I don't think CPRK will be getting top dollar for copper concentrate right off the bat, and I don't think CPRK necessarily drove a hard bargain. Rather, I think they got an acceptable deal done. The nice thing is that CPRK has a buyer. One day CPRK may be in position to sell con to someone closer to the consumer, but right now CPRK needs cash flow and Sempra can provide it, if CPRK can deliver the concentrate.
It's like selling gold to a pawn broker. The pawn broker will buy gold all day long at a price that they can turn around and sell it all day long. I hope someday CPRK will get into a position to sell to markets closer to the consumer. That will be a long ways down the road IMHO.