You didn't say what the 50 stands for - pesos (using a $ sign, like a U.S. dollar) or centavos.
Either way, its value is very low. Mexico's currency was affected by severe inflation during the 1980s and the value of the peso fell to about 1/100 of a U.S. cent. The current peso is a so-called "new peso" worth 1000 old ones. That means your coin is worth 0.05 new pesos, and at current exchange rates one new peso is about a dime.
Probably a slight premium to the spot value of silver (about US$9.40 as of 11/19/2008).
It depends on the face value and condition. One 'plata libertad' coin can be worth up to US$34.00
200
Jodan 1991 silver coin
In good condition, the spot (market) price of an ounce of silver plus ~ $4
Cool coin. Mexico issued this large 25 pesos coin only in 1968. It is made of 72% pure silver by law (0.720 ley) and shows the national coat of arms of Mexico (eagle, snake, cactus). Each coin contains 0.521 troy ounces of silver, so look up the current value of silver and multiply by 0.521 to get the base value (BV) of this coin. The coin cannot be worth less than BV.
No such coin exists. Silver dimes were produced for circulation dated 1964 and earlier. Silver proof sets started being sold to collectors starting in 1992. There is no 1985 dated silver dime.
The value of a silver coin is always changing because the value of silver is always changing. See the related link below for a silver coin value calculator. This does not give the actual value of the coin but it does give the value of the metal used to make the coin. This is know as the melt value.
Metal value depends on the purity of the silver and the weight of the coin. Numismatic value depends on the condition of the coin.
They issued several different designs for this coin in 1985, but they each retail at $18.50 or silver value - whichever is highest at the time. (Silver content is .5677-ounce)
A Silver Eagle Bullion coin is 99.9% pure silver and weighs 1 oz per coin. The stamped value of the coin is U.S$1 however the intrinsic value of the coin is the value of the silver within that coin. If the the spot price of silver was $30oz at the time of checking, then the value of that coin would be worth at least $30 or more.
The U.K. does not issue quarters so I assume you have a Canadian coin. 1985 Canadian quarters are minted from nickel, not silver, and are in general circulation. If that describes what you have it is worth only face value, about 23 cents U.S.
The value of an uncirculated 1999 Silver American Eagle Littleton coin fluctuates based on the market for silver. However, a coin can be expected to have a value of about $40.
This coin is most likely worth the silver value.