Coin prices are not as clear cut as other commodities since the grade of a coin greatly affects its appeal thus influencing its demand and value. And since value can be subjective we strive to determine relative value based on price trends, not necessarily absolute value. There are too many variables between grades and the price of a coin is only as accurate as what someone is willing to pay in real dollars. The key is to determine the trend direction in addition to relative value so you can get a long term sense of future value.