How much is a 50 peso coin 1984?

A 50 peso coin is essentially valueless, because the peso of 1984 was converted to a new peso currently in use, and older peso coins removed from circulation. A person who walks into a coin dealer in Mexico City can find these coins at low prices (one or two pesos, maybe 20 US cents) just because the artwork on the coin is of historical interest, but the occasional E-Bay posting of these coins with prices of $5 or $10 represents a huge markup on what is essentially a valueless (although pretty) coin.