The 1904-S Barber Half Dollar is one of several very tough San Francisco issues of this series. The mintage of 553,038 was below average, and most pieces circulated heavily. This issue has the lowest population in Mint State grades of all regular (non-variety) coins. The finest known are 2 or 3 different pieces graded MS-67 by PCG or NGC, which have sold near the $100,000 mark in recent times.
Even in circulated grades, the 1904-S Barber Half remains a key date of the series. Coins in problem free condition can be difficult to acquire and are in constant demand.