It is tempting to call the 1931-S Mercury dime a sleeper. That is the case in some grades, but it is in a group where low mintages and seemingly low prices are common and it is unlikely that anyone has overlooked the entire group. So while probably not a sleeper in the normal sense, it might be more correct to suggest that the 1931-S is simply a very good value.
A major reason why the 1931-S is a bargain is because its mintage is at just 1,800,000. However, all Mercury dime mintages at the time were low. The 1931-D had a mintage of 1,260,000, and the 1930-S was almost identical to the 1931-S at just over 1.8 million.
The Great Depression was the reason for the low mintage. It was not yet the worst days of the Depression, although they were not far away. When the economy is weak, so is demand for new coins and that means mintages drop, sometimes dramatically. That had been the case in the 1870s and again in the 1890s, so it was no surprise in the 1930s. This didn’t affect just the dime – in some cases there were no mintages at all.
Normally you would think that low mintages in a depression would make the dates even better than the mintages suggest since people would have no interest in spending what limited funds they had on coins. That might have been true to a degree at the time the 1931-S was released, but the Great Depression was a very unusual situation when it came to coin collecting.
In fact, interest in coin collecting increased even during the bad times. It was largely because the 1930s would see the introduction of albums and holders for collections that had never been available in the past. That, along with increased promotion on the part of dealers like B. Max Mehl, encouraged collecting. There was also a continuing production of commemoratives, and every little bit helped to create new collectors.
Grading service totals make it readily apparent that there was an enormous increase in the saving of new issues starting in the 1930s. That explains why assorted dates from the 1930s are not as expensive as we might expect, especially in Mint State.
This is true with the 1931-S Mercury dime, but slightly less so than in other low mintages from the 1930s. The 1931-S is at $96 in MS-60, and the lower mintage 1931-D is just slightly more at $100. In MS-65 the 1931-S lists for $275, and the 1931-D is at $285.
It’s a different situation in MS-65 with full split bands. The 1931-S is $2,100 as it is legitimately tough, while the 1931-D is just $350. The 1931-S, or at least those that have survived to present day, does not regularly have full split bands. We cannot be sure of the reason for this. It simply could be bad luck since collectors saved the coins, but the ones they saved were not the highest quality.