supply and demand...in this case especially demand. the more folk that want to collect a thing drive up the price of that thing based on its supply. more...by a factor of maybe 10... folk choose to collect U.S. coinage, prices go up. Canada also floods the market every year with all sorts of non-circulation silver, gold and commemorative coins, made exclusively for the collector market. these coins often have a nominal value..$1...$20...$100 etc. but the mint slaps a premium on them that doesn't keep its value 10 years down the road...and thus they revert to little more than bullion value. there is a lot more than my simplistic explanation, but these are the basics....condition, mintage and market.