Let's say you found a 1964 90% Roosevelt silver dime today. How much would it be worth?
First, go to Kitco.com, which gives you an ever-changing up-to-the-minute price guide for silver (and most other precious metals too). At this moment (13 January 2011 19:44) price is 28.78 per ounce. US silver coins are 90% silver. According to 2005 North American Coins and Prices, this coin weighs 2.5000 grams and contains 0.0724 oz. of silver. Multiple just the silver content of the coin (0.0724) time the current price of silver ($28.78) and you should get $2.0836, which is the current price for the bullion (silver) value of this common coin in almost every condition you are likely to find it in the ground. That is also between 20x and 21x face value (10 cents).
The same principals apply to any common-date US silver coin without a lot of metal loss due to coin handling/wear.
Is the price always a function of the value of the coin? Not necessarily. Take an 1942P Jefferson Silver war nickel. This nickel weighs 0.3500 ounce in unused condition. It contains 0.3500 oz. of copper, and 0.0563 oz. of silver. Substituting in the equation above, the silver value alone of a 1942P silver war nickel would be $1.62/coin, or slightly higher than 32x (32 times) face value.
Many newspapers give a weekly quotation for $1,000 bags of silver coins. It's fairly easy to work out how many times face value such bags are worth.