Geography
Why would an army wagon train with $ 1,000,000 dollars (1865 value, $ 15,462,453.99 2018 value) in gold be travelling through the Staked Plains, one or two hundred miles north of the real Phantom Hill, Texas, in 1865?
The mighty Comanche, along with their Kiowa and Kiowa Apache allies, lived in an area with the Staked Plains as the center, and raided and terrorized a much wider area, and continued to do so for almost a decade after 1865, until the Buffalo or Red River War of 1874-1875.
So in 1865 only large expeditions that could fight off attacks by tens or hundreds of Comanche warriors, would dare to travel across the largely uncharted and unexplored Staked Plains.
The gold mined in California was sent east by ships around Cape Horn or through Panama or Nicaragua, until the transcontinental railroad was completed in 1869.
Shipments of gold to pay annuities to Indian tribes or army payrolls would be sent along normal trails used by travelers in the west. And of course none of those trails went through the Staked Plains which were the home and headquarters of the dreaded Comanches, or as one character in the movie put it, "Satan's Land".
I believe that in 1865 the pay of an army private was $ 16.00 per month, paid every two months. So $ 1,000,000 would have equaled two months pay for 31,250 US army privates. In 1865 there could have been almost 30,000 US army privates in the west, the largest number stationed there during the 19th century. But they were stationed in maybe 50 or 100 separate posts scattered from Washington Territory to Texas, and from southern California to Minnesota.
So I can imagine that a single shipment on a wagon train might contain the pay for all the troops in an entire state or territory, possibly as much as ten or twenty percent of all the soldiers in the west, and thus maybe $ 100,000 or $ 200,000.
And what form of money were US soldiers paid in back in 1865? Would they have been paid in gold coins? It is quite possible that they were paid in paper money. Anyway, the gold is in gold bars. So it wasn't an army payroll shipment.
And in those days there weren't any trails between important places in the west that went north or south, east or west, through the Staked Plains. Sending $ 1,000,000 in gold to pass that close to Rebel forces was really foolish and risky, while sending $ 1,000,000 in gold with such a small escort through the Staked Plains would have been like giving it to the Comanches as a present, only with a bunch of soldiers getting killed.