Due to the US treasury having minted a huge collection of Morgan Dollar more most of the dates, these coins have been very common and available at low prices in the coin shows. However, there are many dates of the Morgan Silver Dollar series getting rarer because of low minting.
The 1889-CC dollar which minted around 200,346 coins, 1893-S which minted 100,000 coins and 1895 Proof which minted 880 Proofs and 12,000 Business Strikes are considered key dates and are highly valuable in the Morgan Dollar series.
Some of the coins of the 1895 Proofs, disappeared over the time, decreasing the number of Proofs. As a result the 1895 Proofs have become rarer and the most valuable. The value of 1895 Proofs is estimated to be around $80,000 as per the 2010 Red Book. But the 1895 Business Strikes were kept away from circulation.
Many of the dates of Morgan Dollar Series are considered as condition rarities. Such coins are of common dates, have low premium and are well circulated. The value of these coins is very low, when compared to the silver content of the coin, though mint condition coins do bring a fair price. Some of the coins which are considered to be condition rarities are 1884-S, 1886-O, 1892-S, 1896-O, 1897-O and 1901.
Another reason for varying value across the Morgan Dollar series was uneven circulation of the coins. Some of the coins of the series gained fame in commerce, which is why they received huge circulation. While some of them were rarely used in everyday commerce due to which they were never exposed to circulation. Further, many uncirculated coins stored in the U.S treasury were melted under the Pitman Act of 1918.
One such example is of 1884-S. This date of the series found extensive use in commerce and as a result was in heavy circulation. Very few of them were preserved in the U.S treasury. The few uncirculated coins left in the treasury were melted under Pitman Act. This made the mint condition 1884-S silver dollars great rarity.
Certain dates of the Morgan Dollar series went rarer while a few remained common. This made the uncommon Morgan dollars attain high value while the common ones gained a very low value. Red Book stated MS67 1895-O as the Morgan dollar which was sold at the highest price of $575,000 in November 2005, Heritage Auction.
Alan Brenner maintains 1900SilverDollar.com. A site dedicated to the Morgan Silver Dollar including the 1893 Dollar.