Where can you find out about a 1981 Estados Unidos Mexicanos thick silver 20 piece and what is it worth in good condition but with a slight dent on its edge?

This is not a silver coin -- it is actually made of a copper/nickel (like a U.S. nickel) Last 3 listing on eBay : One passed at a 10 cent price One sold at a $2.50 price One passed at a $5.00 price