You need to know the coin's mint mark and condition. The mint mark, if it exists, is found on the reverse under the eagle. No mark = Philadelphia, O = New Orleans, S = San Francisco, CC = Carson City. Retail values for all mints except CC would range from maybe $15 in average circulated condition up to $20 or so if the coin has almost no wear. An 1880 Carson City dollar, however, would run from $90 to $200. If you do have a CC dollar, you'll need to have it examined by a reputable dealer or other expert since the mint mark is frequently counterfeited. Finally, as you may have noticed by reading some of the many, many other posts on this site, the motto E Pluribus Unum appears on ALL U.S. coins so it doesn't really identify a coin.