Murder in the lower classes
Since a sizable percentage of murders in the lower classes are gang related I'd suggest your premise above is false MMI.
If anything the economic motive is more commonplace in the lower classes, where a sizable percentage of killings are related to the drug trade. A crack dealer in the inner city has a 25% chance of surviving the next 5 years. Three in four crack dealers will die over that five year period, the vast majority in gang related homicides (aka turf wars). Rival gangs protecting territories related to the drug trade is without a doubt an economic motive.