The 2 player flavor of Mercator adds the ability to receive 2 coins when the 5 coin space is taken.Maybe the two player rules are different... but in the normal game, there's no cost to placing a die there unless someone else already has a die there. In general, you never pay a cost to place a die unless there is a die already in that spot.
My (limited) experience is that the 5 coin space is rarely taken in a 2 player game. Because the spot is filled by a dummy die, it at best nets you one extra coin (relative to the 3-coin bonus action) - if and only if you place a '1' die there.
Even when that opportunity does present itself to Mercator's opponent, it seems unlikely that they will feel that the extra coin justifies giving 2 free coins to Mercator.
So, it seems to me that this part of Mercator's ability is pretty close to pointless. Am I missing something?