Cormac McCarthy based Glanton's gang on a real-life group of marauders who were hired by local Mexican leaders to hunt indigenous people (i.e., Native Americans). Like Glanton's gang does in the novel, these real-life scalp-hunters went out into the frontier and brought back Indian scalps as receipts for payment. It is useful to understand this historical trend in reading Blood Meridian, since McCarthy's bleak view of human nature is heavily influenced by his vast historical research.
The business of trading scalps for gold might not have been new to this particular era of American Expansion. Although scalping was a common practice during 1835-1880s (the era in which Blood Meridian is set), there is archeological evidence that scalping had been occurring in North America from as early as 1325, most famously in the Crow Creek Site in South Dakota. There were also scalp bounties offered in the original colonies, like Massachusetts and Pennsylvania, back in the 1700s.
However, the scalping industry became more formalized and widespread when Mexican governments began using various mercenaries to keep their citizens safe from the Apaches and Comanches. It is this era of institutionalized violence that McCarthy uses as his setting. The Native Americans would come into an unorganized Mexico, wreak havoc, steal livestock, and then retreat back to the United States. In 1835, the Mexican state of Sonora decided to retaliate. The local government decided to offer 100 pesos each for scalps of "braves." Industrious Americans recognized the profitability of this new business, and the opportunity was especially appealing to those who were trying to earn enough money to get to California (like many members of Glanton's gang). After that, the murder business grew progressively more profitable as the Apaches and Comanches continued to decimate Mexican communities.
After the Mexican-American War, scalp bounties went up in price. Despite some efforts to regulate the killing (some of the Mexican states tried to prevent fraud by creating standard definitions and practices), the scalping business remained profitable until the 1880s, when the unpredictability and brutality finally took its toll. It turned out that the mercenaries often became a threat themselves, just like Glanton's gang does in the novel.
Nevertheless, scalping made a reappearance during the American Civil War (1861-1865). "Bloody Bill" Anderson, the infamous pro-Confederate guerrilla leader, was known for dangling scalps of Union soldiers on his saddle to remind his opponents of his ruthless abilities. Of course, unlike the mercenaries represented by Glanton's gang, Anderson was not paid for his wares.