1919 Mercury dimes were struck in large numbers and remain substantially plentiful today, particularly in the lower circulated grades, meaning they are quite affordable for coin collectors on virtually any budget. However, the fact that they are inexpensive in the lowest grades does not necessarily translate to a low cost for buying higher-grade 1919 dimes.
Taking a look at the mintage figures for all three 1919 dimes (those struck at the Philadelphia, Denver, and San Francisco mints, that is), may provide you a clue as to why the D- and S-mint Mercury dimes are considerably more expensive than the Philadelphia issue in the upper grades.
1919, 35,740,000 minted; $4 in Good-4
1919-D, 9,939,000; $7
1919-S, 8,850,000; $6
While tens of millions of dimes were struck at the Philadelphia mint, only about half of that figure was struck at the other two mints, collectively. And, as was the case for most coins early in the 20th century, 1919 Mercury dimes, especially those from the branch mints, were not saved in vast quantities. Therefore, they are quite expensive today in MS-60 and above – each about $350 in that grade. Even in the grades of XF-40 and AU-50, the 1919-D and 1919-S dimes are somewhat pricey, at around $30 in the Extremely Fine grade range and closer to $75 for an About Uncirculated specimen.