It depends on condition. One 'plata libertad' coin like you just described would be worth up to US$34.00
Yes, there are many silver Mexican coins, however, when compared to other countries, Mexico used lower purity silver in their more modern coins and often times different coins had different cut-off dates for silver. For example, until 1918, Mexican Peso coins were made out of 90.3% silver, in 1918 that was reduced to 80% silver, in 1920 it was reduced again to 72% silver, in 1947 it was reduced yet again to 50% silver, in 1950 it was reduced to 30% silver, and in 1957 the coin was only 10% silver until 1970 when it was replaced with a copper-nickel coin containing no silver. Mexico also has many modern bullion coins struck in silver.
it is illegal in Mexico to take coins to other countries
Of course. And "coin silver" only refers to the US standard of 90% pure silver coins. Most other countries (especially in the British empire) used to use sterling silver. Coins have used many different alloys, for example, post WWI Canadian coins are 80% silver, silver UK coins dated 1920-1946 are 50% silver, some ancient "silver" coins are known as billion coins and they contain very little silver, and Mexico issued a 1 peso coin in the late 1950s and 60s that was only 10% silver!
All New Zealand silver coins from 1933 to 1946 had a silver content of 50%. From 1947 onwards, all New Zealand "silver" coins were made from a copper-nickel alloy.
1930 50 centavos
The last silver 100-peso coins were minted in 1977, so that is the approximate date when Mexico dropped off from the silver standard.
No British coin has contained any silver since 1946. From about 1919/1920 to 1946, all British silver coins were minted with 50% silver. Prior to 1919/1920, all British silver coins were made from sterling silver (92.5% silver).
The British silver coinage was degraded from 1919/1920 from sterling silver to 50% silver. All British silver coins from 1919/1920 to 1946 inclusive had a 50% silver content. From 1947 onwards, all circulating British "silver" coins were made from a copper-nickel alloy.
The government stopped putting solid silver coins out in the late 60's. Any 50 cent piece past 1969 will not be silver.
Oh, dude, they stopped putting silver in Mexican coins in 1979. It's like they were all, "Nah, we're done with the shiny stuff," and switched to cheaper materials. So, if you're looking for some silver in your money, you might need a time machine.
Australian silver coins (5, 10, 20 and 50 cent) are composed of 75% copper and 25% nickel. There is no silver in any circulating Australian coin.
If it is a collection of 50 cent coins, it could be worth a lot, or not that much. It depends on the dates of the coins and the condition. 50 cent coins from before 1965 are worth more because they are 90% silver.