This is probably every coin collector's "number-one question" and also one of the most difficult to answer. Here's why:

All legal tender coins have a face value, the amount the coin is intended to be worth when issued. Every coin is worth at least that much, which means that no matter how many 1964-D nickels you find, each one will always be worth five cents at a minimum. The caveat to this is that in some cases coins are not legal tender. This is not a problem with US coinage, all of which can theoretically circulate at the value stated on the coin; however, coinage from around the world may have been devalued or even demonetized entirely.

All coins have an intrinsic (or bullion) value. This is the value of the metal the coin contains, and explains why we don't see gold dollars circulating any more. The intrinsic value of most circulating coins is negligible, hopefully less than the face value, but if the coin contains gold or silver, its intrinsic value is likely to be far higher than its face value.

All coins have a numismatic value. This is the value a coin has to collectors. A common circulating coin in its country of issue is likely to have a numismatic value exactly equal to its face value; however, for less common coins, older coins, and world coins, this numismatic value is likely to be somewhat higher than the face value. (See ?Lyon?s Law of Currency?)