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If you are interested in collecting coins, it's important to be able to have a rough idea of how much coins are worth. Knowing how coin values are determined will enable you to find good deals, and ensure that you don't get cheated into paying too much money for a coin with a low worth.
One major factor in determining coin values is the law of supply and demand. If there are many coins of a particular type available, that coin will not be worth much. On the other hand, if only a very few coins of that type are produced, the coin values will rise. This is why a completely normal-looking copper penny minted in 1943 is worth about $200,000, whereas a 2,000 year old Roman coin may be worth less than $100 - because thousands upon thousands of Roman coins were minted, but only 40 pennies produced during war-time 1943 were made out of copper.
Even among coins with a similar number of copies in existence, some have a higher worth than others. This is because some coins are in higher demand, driving up the coin values. Coins may become popular because they are particularly lovely to look at, because they are part of a topical set that is often chosen by collectors, or because they have a certain historical significance.
Some coins are made out of precious materials like gold bullion or platinum. These coin values are less volatile because the worth is guaranteed in part by the material. A gold bullion coin, for example, is usually worth more than its melted weight, but it is never going to be worth less.
The final major factor in determining coin values is the grade or classification of the coin. The more wear and tear that a coin has undergone, the less value it is going to have. This is why uncirculated coins are usually more valuable than coins that have been passed from hand to hand. Uncirculated coins have always been kept in the very best of conditions, making their value much higher. A coin in flawless condition may be worth hundreds of times more than a low-grade version of the exact same coin.
Now that you understand the basic factors that influence coin values, you have a better grasp of which coins may have real value and which will be worthless. In order to get a ballpark estimate of the value of any coins you might have, you will first need to determine its grade or classification. You can do this by comparing your coin's condition to a published list of guidelines. Then look up the value of a coin in that condition in a book such as The Standard Catalog of World Coins, which should be available in most public libraries. If you need to know the exact amount that your coin is worth, you should take it to a coin dealer and let him or her evaluate it for you.