How much would an 1891 E PLURIBUS UNUM one dollar silver coin be worth?

The value of your 1891 Morgan Dollar will depend on its condition and if it has a mintmark (on the reverse, above the "DO" in DOLLAR").

Current values :

No mintmark : well-worn -- $8 lightly worn -- $10 uncirculated -- $35

"CC" mintmark : well-worn -- $65 lightly worn -- $120 uncirculated -- $250

"O" mintmark : well-worn -- $8 lightly worn -- $11 uncirculated -- $90

"S" mintmark : well-worn -- $8 lightly worn -- $12 uncirculated -- $35

An online retail price guide for other Morgan Dollars can be found here : http://www.numismedia.com/fmv/prices/mordlr/pricesgd.shtml

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Dan Moore The Working Man's Rare Coins http://www.workingmancoins.com Price guide

Also please note that ALL U.S. coins bear the motto "E Pluribus Unum" ("From many, one") so this is not a distinguishing characteristic of a coin. The denomination, date, mint mark, and condition are the most helpful characteristics to start with.