Manganese is not a rare mineral. One does not mine manganese nodules for the manganese. Mining them would create an enormous surplus of manganese that would get dumped in waste piles.
I wasn't making a judgment here. I was just pointing out that most of the manganese ends up in the waste stream. One would not be mining these things for the manganese (although what manganese that could be sold would be sold.) In a world mining manganese nodules, the price of manganese would be very low.