40 cents.
These rates change hourly, but as of today the 22nd of July 1320 Mexican Pesos are worth around $99.50USD. Becareful!! ONLY NEW PESOS are accepted for exchange. Any coin or bill made before 1992 may have a collector value but they are no longer circulating currency.
about 230 US dollar, the current rate is about 13 MXN per USD However, that's only true for pesos dated 1992 and later. Mexico revalued its currency in the early 1990s due to inflation, and older pesos are worth 1/1000 as much.
During the 1980s Mexico endured a period of high inflation that greatly eroded the peso's value. In 1992-93 existing pesos were replaced with "nuevo pesos" (new pesos) at an exchange rate of 1000 to 1. That means your coin is equivalent to 2 new pesos, or about 20¢.
Mexican currency is denominated in pesos rather than dollars, but they use the $ sign to indicate pesos, so it causes a lot of confusion. You'll need to post a new question with your coin's date and a description of the image on it. The date is very important because Mexico changed the value of its currency in 1992-93. "Nuevo pesos" (new pesos) were issued to replace old ones at a 1-to-1000 exchange rate. That means if you have a coin worth 20 nuevo pesos, it would exchange for about two U.S. dollars. However, if it's old pesos, it would be worth only one-one thousandth as much - i.e. just two-tenths of a cent.
Mexico revalued its currency in 1992 due to severe inflation. "New pesos" were issued to replace the then-current ones at a rate of 1000 to 1, so your 20,000 old-peso note translates to 20 modern pesos, or about $1.50 as of 10/2009.
You can't. Due to hyperinflation in Mexico the old peso (Pesos issued until 1993) was replaced by the "new" Peso at the rate of $1000 old pesos to the new. So your bill would be 50 new pesos. However, Mexican banks stopped redeeming old pesos for new in 1996 so your bill is just a collectors item and could be sold on eBay and the like, but can't be redeemed for Mexican Pesos.
Mexico experienced runaway inflation during the late 1980s and by 1992 was forced to withdraw all bills and coins because they were becoming effectively worthless. They were replaced with "nuevo pesos" (New Pesos) at a rate of 1000 old = 1 new. Unfortunately that means your bill is only worth 1 new peso, which is about 10¢ U.S. at current exchange rates.
curling
Mexican coinage lost enormous amounts of its value during the 1970s and 1980s due to hyperinflation. In 1992-93, the old coins were replaced with new ones in the ratio of 1000:1. That means 20 old centavos are worth 0.020 new centavos. With the current exchange rate of 1 new centavo = roughly US$0.001, I'm afraid your coin has value only as a curiosity piece.
100 new pesos=$9.3217 American dollars 100 old pesos (before 1992) = slightly less than 1 cent.
Most often "Peso" refers to Mexican currency. However Mexico is not the only country that uses pesos. Some other countries that use pesos are: Argentina, Chile, Cuba, and Uruguay. Ask another question and include the following information: date, country of origin, and the denomination. For example "What is the value of a 1978 Mexican peso" or "What is the value of a 1978 Cuban peso". If your coins is from a different country or denomination just include the correct information.
Runaway inflation during the 1980s reduced the value of the peso to almost nothing. Mexico was forced to withdraw its old currency and replace it with "new pesos" in 1992 and 1993. Unfortunately the old pesos traded at about 10,000 to the dollar so 1000 old pesos are worth around a dime.