Although you seldom see them, we do have a dollar coin in circulation today. For about the past one hundred years they have been rare in the United States, where people seem to have an unexplained resistance to using them, although most countries in the world have coins for their basic unit of currency. The dollar coins being minted today are issued with a different presidential portrait once every three months. Naturally, a coin with the face value of one dollar today is worth considerably less than one of the old silver dollars of yesteryear.
Those old silver coins are highly prized by coin collectors, who regard the as valuable not only for the intrinsic value of the silver, but for the rarity and the beauty of the coins, so a collector may find that some of his old US dollars are worth more than many of a gold coin from his collection. This is not at all surprising, since some coins issued as collectables, even gold ones, are mass produced for consumption by amateur collectors, and while valuable for their precious metal content, would not really lose any value if they were melted down.
Compare this with the morgan dollar , the standard US silver one dollar piece minted between 1878 and 1921, the price of which can depend heavily of when the dollar was minted,some years being more rare than others. The price also depends on whether or not the coin in question was circulated or not.