big holdings - discount to market value??

Just wondering when a company has a big holding in another (ie. 20 %) is it normal to discount this holding from market value as you would be unlikely to be able to sell the whole holding for this value without bringing the price down??

Or alternatively do big holdings get valued more than market value as its hard to get that big a chunk so a potential buyer may pay more to get the 20%??

Just pondering/wondering??