If you consider the metallic weight and purity of a coin and compared it to today's market price of gold or silver, then you'd get a rough idea of its price in US dollars. You can use Argmaur.com or other resources that display current precious metals market prices for help.
If you have a rarer, older, or higher grade coin, then obviously you can add value to the market price of the metal. The easiest way to determine the current going price for your coin is by consulting Ebay--yes, Ebay.
Because Ebay is a free market of thousands of coin buyers and sellers, the result is reasonable prices. In essence, buyers will only buy coins that are deemed fairly priced. Sellers, on the other hand, will ask for the lowest price just in order to make the sale and beat the competition; but few sales are ever made below precious metals market prices.
Thus, the tug of war between fair prices demanded by buyers and sellers creates a stabilized and accurate price market. You'll discover that the average sale price for a specific coin on Ebay will be similar to prices found at major coin dealers and auctioneers such as APMEX, Kitco, and Heritage Auctions.
In addition, you can use countless coin buying guides and books; however, Ebay coin sales represent not what a book says a coin is worth, but what people are willing to pay for a coin.