What is the value of an 1888 US silver dollar?

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 an O or S. (The M on the front of the coin is the designer's monogram and not a mint mark.)

Numismedia lists the following approximate retail values as of 05/2011:

No mint mark (Philadelphia):

Very worn condition - $38

Moderately worn - $39

Slightly worn - $40

Almost no wear - $42

Uncirculated - $50 to $8,030 depending on quality

"O" mint mark (New Orleans):

Very worn - $38

Moderately worn - $39

Slightly worn - $40

Almost no wear - $42

Uncirculated - $50 to $12,500

"S" mint mark (San Francisco):

Very worn - $77

Moderately worn - $144

Slightly worn - $161

Almost no wear - $204

Uncirculated - $264 to $34,810

DISCLAIMER: The retail values quoted are the best available as of the date shown, but may vary significantly due to changes in the precious metals market. 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.

FAQ's about Morgan dollars

E Pluribus Unum

The motto E Pluribus Unum (Latin for "Out of Many, One") is on ALL $1 coins minted since 1878 so that's not specific enough to ID any one coin.

Silver Eagles

Silver eagles are bullion coins minted since 1986 for sale to collectors but not for spending. Morgan dollars were made for use in ordinary circulation just like dimes and quarters.

Please see the attached link.
Assuming the coin is circulated and has no mintmark, the 1888 Morgan dollar is a very common coin with retail values of $23.00-$31.00 depending on the grade of the coin.