The 1878-S is readily available in VF and XF. It becomes scarce in AU. Akers comments may have been true in 1980, but not by 1993.
In August 1993, Coin World Trends for an 1878-S Eagle in AU50 was $9500. Less than three years later, the Trends price had fallen to $1900. Two morals: 1) don't place too much faith in published prices, and 2) don't pay a huge premium for a higher grade.