A 1891 Morgan Dollar in good condition is worth: $ 20.00.
The usual name for this series is "Morgan dollars", after the designer George Morgan. Look on the back of the coin to see if there's a small mint mark letter above the DO in DOLLAR. It may be blank or there may be a CC, O or S.
Numismedia lists the following approximate retail values as of 11/2010:
No mint mark (Philadelphia): Very worn condition - $20
Moderately worn - $21
Slightly worn - $22
Almost no wear - $29
Uncirculated - $53 to $56,250 depending on quality
"CC" mint mark (Carson City): Very worn - $69
Moderately worn - $91
Slightly worn - $98
Almost no wear - $187
Uncirculated - $330 to $53,130
"O" mint mark (New Orleans): Very worn - $20
Moderately worn - $21
Slightly worn - $22
Almost no wear - $42
Uncirculated - $130 to $41,880
"S" mint mark (San Francisco): Very worn - $20
Moderately worn - $21
Slightly worn - $22
Almost no wear - $29
Uncirculated - $58 to $14,380
DISCLAIMER: The retail values quoted are the best available, but may vary due to market fluctuations. Also the wholesale (buying) price of a coin will be less than the selling (retail) price. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.
ALL U.S. coins carry the motto E Pluribus Unum so that does not help to ID the coin.
7-18-11>>> Assuming the coin is circulated and has no mintmark, the 1891 Morgan dollar is a high mintage common date, retail values are $36.00-$45.00 depending on the grade of the coin.