100 francs used to be a boatload of money. France first issued 100 franc coins in .900 fine gold in 1855. They were big, bold, and contained 0.933 troy ounces of pure gold. At today's (Oct 2010) market price of gold, around $1300 US dollars per ounce (look it up on kitco.com), that's 0.933 x 1300 = $1212 US dollars of gold in one 100 franc coin.
The value went down and down. In 1929 a few 100 franc coins were minted in gold, but smaller, with only 0.189 troy ounce of gold, or about $245 worth of the precious metal. Then, in 1954, gold was dropped all together and 100 franc coins were minted in copper-nickel, with $0 worth of precious metal.
To me, this is sad. The world's monetary system has been completely changed to paper, metal, and now computer electrons with no intrinsic value. (Sorry for the political commentary!)
On a brighter note, France decided to issue 100 franc coins in 0.900 fine silver starting in 1982. So, here is the saga of modern French 100 franc coins:
1954 to 1958: copper-nickel, worth only a few dollars to collectors if in excellent shape.
1959 to 1981: no 100 franc coins minted.
1982 to present: silver, with roughly 1/2 troy ounce of silver in each coin (actual silver weight varies from about 0.4 troy ounces to 0.65 troy ounces depending on specific dates)
There are also random years where 100 franc coins were issued in platinum, gold, and palladium. The are proof coins minted exclusively for collectors and bought and sold as collectibles, not as real money.
To get the value of modern silver 100 franc coins, multiply their weight in troy ounces by the current price of the precious metal they contain (see kitco.com).