Bullion Value

Gold, silver, platinum, and palladium content coins have an inherent bullion value due to their precious metal content. This is called their bullion value.

Many older coins are worth more than their bullion value. The bullion value sets the base price the coin will ever be sold for. It also gives you another way to value the coin if for some reason the coin lost its numismatic value.

The other option bullion value gives you is that if you decide you need to liquidate quickly you can sell the coin for the bullion value instead of numismatic value.

Gold

Gold is the most popular of the precious metals as an investment. Gold coins are very appealing to investors due to the high value of the gold content as well as the possible numismatic value of many gold coins. This is especially true when it comes to old gold coins.

Silver

Silver coins are very popular as well.  This is due largely in part to the price of silver bullion in comparison to gold bullion. Silver coins are much cheaper then gold coins, if you are only looking at bullion value of the coin.

For example a Silver Eagle, which is one ounce of .999 silver bullion, will cost you around $40.00 USD, while a Gold Eagle, which contains 1oz. gold bullion will cost around $1,800.00 USD. This makes it easier for people who want to invest small amounts over time to still be able to invest in precious metals.  It also does not require the investor to have large sums of money to invest all at once either.

The Grade of a Bullion Coin

The higher grade of the coin the harder it is to find. The minting, storage and transport process used for manufacturing coins causes a lot of incidental damage to the coins surface. Damage ranges from nicks, dings, and scratches -- to general wear. The higher the grade, the rarer the coin.

The strike of a coin is important, as well. The strike, if well struck and distinct, can add value to a coin. Weak struck coins are less desirable due to the design being less impressive unless it is an error coin.

Proof-like condition adds value too, since this is a more rare condition for a coin. Coins that have DMPL (deep mirror proof like) qualities are extremely rare since the conditions for striking such a coin do not occur very often.

The grade and eye-appeal of the coin are an important measure of its value. Even the placement of a scratch or blemish on the coin can affect the value. There is a 70 point grading scale called the Sheldon scale that is the scale used to grade a coin.

The Rarity of Coins

The rarer or scarce coins tend to have higher values. Scarcity is determined by the mintage per year, per mint mark, and per grade, plus any special "varieties" or "types" that were created that year.

There are coin books that show all the mintages, dates, mint marks, and variations of coins as well as their values. These books can help you understand how these factors work together to change a coins value.

The more valuable coins have lower mintage numbers. You need to study the mintage and prices of a coin series to understand the relationship. This valuation is measured separately for each series, there is no one rule for all coins. 

Common date coins with a large mintage are less likely to rise in value as compared to a more rare date coins with low mintage. Coins that exist in very scarce numbers are considered highly desirable. 

Error coins are rare coins that are valuable because they have a subtle difference or differences that set them apart from other coins. Some of these differences include things such as double stamping a coin, partial stamping, lightly stamping, which makes the coin a little bit unusual.

These can be very subtle differences or very obvious differences. These error coins are prized among dealers because they extremely rare and bring a lot of money, so they are worth adding to a coin portfolio.

The Popularity of the Coin

A coin series needs to be popular among collectors; this will help guarantee value in the future. This is only one of many aspects that determine the value of a coin. Popularity also helps to guarantee that there will be buyers willing to pay money for the coins if you were to sell the coin.

Some coin series are popular because of their "looks" some are popular because of their rarity, some because of their uniqueness, and some just because of sentimental value.

Collectors tend to favor certain series and this tends to drive the values up. For example the Krugerrand is the most popular gold coin on the market with over 46 million ounces of gold sold.