The huge difference is the condition of the coin and mint mark (where the coin was made), some are much rarer than others. A nearly brand new (Uncirculated) coin can be worth orders of magnitude more than a very worn one. As an example, I will pull out my 1987 Redbook (Which is obviously very out of date but will do for an example). OK 1878 isn't a good example, there are 5 error variations than range in value, so I'll use the basic as an example:
1878 Morgan Silver Dollar, "S" mint mark (made in San Francisco) was worth $17 in VF-20, $21 in EF-40, $27 in AU-50, $70 in MS-60, $120 in MS-63 and $800 in MS-65 (those numbers are the condition grading of the coin, the higher the number the newer it looks.
1878 Morgan Silver Dollar, "CC" mint mark (made in Carson City) was $35 in VF-20, $42 in EF-40, $55 in AU-50, $130 in MS-60, $200 in MS-63 and $850 in MS-65. Note the difference in value depending on the mint mark.
Again, these are 1987 prices, do not go by them!
Also, do not attempt to clean your coins with any polish or the like, this will ruin their value.
Best bet is have a local jeweler or coin dealer take a look at them, they can give you a rough idea of condition and value.
