How much is a 1986 fifty dollar peso coin worth in America?

There is no such thing as a "dollar peso". Mexico uses the "$" sign to mean peso, and they're 2 different currencies.

Mexican currency was devalued in 1992 at the rate of 1000 old pesos to one new peso, so your coin is actually worth only 5 centavos at current rates. That's less than a penny.