The value of gold coins is ultimately decided by the coin market. Reputable coin pricing guides look at the average prices across a number of coin dealers. These prices are driven by the condition of the specific coin as well as the supply and demand of the market.
Professional Coin Grading Service and Numismatic Guaranty Corporation are two reputable coin-pricing sources that set an average price for a certain coin from prices complied from various sources, including dealer ads, fixed price lists, auctions, major coin shows and website offerings. Specialists and expert collectors provide pricing input as well. Sometimes, these set prices can be adjusted higher or lower due to market corrections.