The1935 Old Spanish Trail half dollar was designed by a collector, L.W. Hoffecker (later, 1939-1941, President of the American Numismatic Association) to be sold to collectors at the relatively high price of $2.00 per coin. It was somewhat of a contrived issue (as were quite a few other silver commems in the 1935-1939 era) and Hoffecker was involved in the distribution, and therefore the profit made, of the coins. Nonetheless, the Spanish Trail was quite popular with collectors at the time of issue and indeed for all of the years following to the present. It was a good design and the quality is generally quite high.
Th Spanish Trail is certainly not rare in an absolute sense. However, the 10,000 coin mintage is very low for a commemorative type issue. Commemoratives of the classic 1892 to 1954 era have always been collected two ways; there is the 50 piece one-of-each-type set and the 144 one-of-each-date-mint mark-and-variety set. For the 50 piece type set, the Spanish Trail has one of the lowest mintages and it has been highly sought after as a key to the series, especially in the days prior to the PCGS Population Reports (pre-1986) as PCGS grading shed light on which coins were saved and which were actually much harder to acquire in top condition than their original mintages might indicate.
The typical Spanish Trail grades MS64 to MS66, and MS67s are also fairly available. The surfaces are usually relatively mark-free as the "high" original price probably caused buyers to take care of their coins. Luster is of the satin type. This is an issue that generally comes very nice.